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HARVARD
MEDICAL LIBRARY
IN THE
Francis A.Countvvay
Library of Medicine
BOSTON
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Prtttrrt0tt lltttwrattg
THE LOUIS CLARK VANUXEM FOUNDATION
LECTURES FOR 1915-1916
Digiti
ized by Google
0f Pnttrrtnti llntti^rBttg
was established in 1912 with a bequest of $25,000
under the will of Louis Clark Vanuxem, of the
Class of 1879. By direction of the executors of
Mr. Vanuxem's estate, the income of the foun-
dation is to be used for a series of public lectures
delivered in Princeton annually, at least one half
of which shall be on subjects of current scientific
interest. The lectures are to be published and
distributed among schools and libraries generally.
The following lectures have already been pub-
lished or are in press:
1912-13 The Theory of Permutable Functions, by
Vito Volterra
1913-14 Lectures delivered in connection with the
dedication of the Graduate College of
Princeton University by Emile Boutroux,
Alois Riehl, A. D. Godley, and Arthur
Shipley
1914-15 Romance, by Sir Walter Raleigh
1915-16 A Critique of the Theory of Evolu-
tion, by Thomas Hunt Morgan
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LOUIS CLARK VANUXEM FOUNDATION
A CRITIQUE
OF THE
THEORY OF EVOLUTION
BY
THOMAS HUNT M)RGAN
PB0FES80B OF EXPEBIMBNTAli ZOOIiOGT IN
COLUMBIA UNIVEBSITT
LECTURES DELIVERED AT PRINCETON UNIVERSITY
FEBRUARY 24, MARCH 1, 8, 15, 1916
PRINCETON UNIVERSITY PRESS
PRINCETON
LONDON: HUMPHREY MILFORD
OXFORD UNIVERSITY PRESS
1916
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HARVARD UNIVERSITY
LIBRARY
JUL 12 1945
/I in
l-ll i
Copyright^ 1916, by
Princeton UniversVtv Press
Published October, 1916
Second Printing, Febniary, 1917
BOSTON MEDICAL LIBRARY
IN THE
FRANCIS A. COUNTWAY
UBRARY OF MEDICINE
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PREFACE
Occasionally one hears today the statement
that we have come to realize that we know noth-
ing aibout evolution. This point of view is a
healthy reaction to the over-confident belief
that we knew everything about evolution.
But there are those rash enough to think that
in the last few years we have learned more
about evolution than we might have hoped to
know a few years ago. A critique therefore
not only becomes a criticism of the older evi-
dence but an appreciation of the new evidence.
In the first lecture an attempt is made to put
a new valuation on the traditional evidence for
evolution. In the second lecture the most re-
cent work on heredity is dealt with, for only
characters that are inherited can become a part
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vi PREFACE
of the evolutionary process. In the third lec-
ture the physical basis of heredity and the com-
position of the germ plasm stream are examined
in the light of new observations; while in the
fourth lecture the thesis is developed that
chance variation combined with a property of
living things to manifold themselves is the
key note of modem evolutionary thought.
T. H. Morgan.
July, 1916.
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TABLE OF CONTENTS
CHAPTER I
A REVALUATION OF THE EVIDENCE ON
WHICH THE THEORY OF EVOLUTION
WAS BASED
PAGE
Preface v
1. Three Kinds op Evolution 1-7
J^ The Evidence for Organic Evolution 7-27
a. The Evidence from Comparative Anatomy 7-14
b. The Evidence from Embryology 1 4-23
c. The Evidence from Paleontology 24-27
S, The Four Great Historical Speculations. .27-89
a. The Environment 27-31
Geoffroy St. Hilaire
b. Use and Disuse 31-34
From Lamarck to Weismann
c. The Unfolding Principle 34-36
Nageli and Bateson
d. Natural Selection 36-39
Darwin
vii
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viii CONTENTS
CHAPTER II
THE BEARING OF MENDEUS DISCOVERY ON
THE ORIGIN OF HEREDITY CHARACTERS
1. Menders First Discovery — Segregation 41-52
2. Mendel's Second Discovery — Independent As-
sortment 52-59
3. The Characters of Wild Animals and Plants
Follow the Same Laws of Inheritance as do
the Characters of Domesticated Animals and
Plants 59-8 1
a. Sexual Dimorphism 61 -64
Eosin eye color of Drosophila 61-62
Color of the Clover Butterfly, Polias
philodice 62-63
Color of Papilio turnus 63
Color pattern of Papilio polytes 63-64
b. Duplication of parts 65-66
Thorax of Drosophila 65
Legs of Drosophila 65-66
c. Loss of characters 66-68
"Eyeless" of Drosophila 66-67
Vestigial wings of Drosophila 67
Bar eye of Drosophila 67-68
d. Small changes of characters 68-70
"Speck'' 68
Bristles of "club" 70
e. Manifold effects of same factor 71
f. Constant but trivial effects may be the
product of factors having other vital
aspects 72
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CONTENTS ix
g. Sex-linked inheritance 73-80
in Drosophila ampelophila 73-76
in the wild species D. repleta 76
in man 77
in domesticated Fowls 77-78
in the wild moth, Abraxas 78-80
h. Multiple allelomorphs 81-84
in the wild Grouse Locust 81-83
in domesticated mice and rabbits .... 83
in Drosophila ampelophila 84
4. Mutation and Evolution 84-88
CHAPTER III
THE FACTORIAL THEORY OF HEREDITY
AND THE COMPOSITION OF THE
GERM PLASM
1. The Cellular Basis op Organic Evolution
AND Heredity 89-99
2. The Mechanism op Mendelian Heredity
Discovered in the Behavior of the
Chromosomes , 99-102
3. The Four Great Linkage Groups of Dros-
ophila ampelophila 103-118
a. Group 1 104-109
b. Group II 109-1 12
c. Group III 112-115
d. Group IV 115-118
4. Localization of Factors in the Chromo-
somes 1 1 8-142
a. The Evidence from Sex Linked
Inheritance 118-137
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X CONTENTS
b. The Evidence from Interference. .137-138
c. The Evidence from Non-Dis-
junction 139-142
5. How Many Genetic Factors are there in
THE Germ-Plasm op a Single Indi-
viDUAL.^ 142-143
6. Conclusions 144
CHAPTER IV
SELECTION AND EVOLUTION
1. The Theory of Natural Selection 145-161
2. How HAS Selection in Domesticated Ani-
mals AND Plants brought about its
Results? 161-165
3. Are Factors Changed through Selection.^ 165-187
4. How DOES Natural Selection Influence
the COURSE OF EVOLUTION?.... 187-193
5. Conclusions 193-194
Index 195-197
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CHAPTER I
A REVALUATION OF THE EVIDENCE ON
WHICH THE THEORY OF EVOLUTION
WAS BASED
We use the word evolution in many ways — to
include many different kinds of changes. There
is hardly any other scientific term that is used
so carelessly — ^to imply so much, to mean so
little.
Three Kinds of Evolution
We speak of the evolution of the stars, of
the evolution of the horse, of the evolution of
the steam engine, as though they were all part
of the same process. What have they in com-
mon? Only this, that each concerns itself with
the history of something. When the astron-
omer thinks of the evolution of the earth, the
moon, the sun and the stars, he has a picture of
diffuse matter that has slowly condensed. With
condensation came heat; with heat, action and
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% THEORY OF EVOLUTION
reaction within the mass until the chemical sub-
stances that we know today were produced.
This is the nebular hypothesis of the astrono-
mer. The astronomer explains, or tries to
explain, how this evolution took place, by an
appeal to the physical processes that have
been worked out in the laboratory, processes
which he thinks have existed through all the
eons during which this evolution was going on
and which were its immediate causes.
When the biologist thinks of the evolution
of animals and plants, a different picture pre-
sents itself. He thinks of series of animals
that have lived in the past, whose bones (fig.
1 ) and shells have been preserved in the rocks.
He thinks of these animals as having in the past
given birth, through an unbroken succession
of individuals, to the living inhabitants of the
earth today. He thinks that the old, simpler
types of the past have in part changed over into
the more complex forms of today.
He is thinking as the historian thinks, but
he sometimes gets confused and thinks that he
is explaining evolution when he is only describ-
ing it.
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TlUSl
ocmi
/pnocrt4r
, ♦COCEfrft
Fig. 1. A series of skulls and feet. Eohippus, Mesohippus,
Meryhippus, Hipparion and Equus. (American Museum of
Natural History. After Matthews.)
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4 THEORY OF EVOLUTION
A third kind of evolution is one for which
man himself is responsible, in the sense that he
has brought it about, often with a definite end
in view.
His mind has worked slowly from stage to
stage. We can often trace the history of the
stages through which his psychic processes
have passed. The evolution of the steam-boat,
the steam engine, paintings, clothing, instru-
ments of agriculture, of manufacture, or of
warfare (fig. 2) illustrates the history of hu-
man progress. There is an obvious and
striking similarity between the evolution of
man's inventions and the evolution of the shells
of molluscs and of the bones of mammals, yet
in neither case does a knowledge of the order
in which these things arose explain them. If
we appeal to the psychologist he will probably
tell us that human inventions are either the re-
sult of happy accidents, that have led to an
unforeseen, but discovered use ; or else the use
of the invention was foreseen. It is to the
latter process more especially that the idea of
purpose is applied. When we come to review
the four great lines of evolutionary thought we
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THEORY OF EVOLUTION
T*m
mm'
W\\ I VlI
t*"f f' ANNS'
Fig. 2. Evolution of pole arms. (Metropolitan Museum.
After Dean.)
shall see that this human idea of purpose recurs
in many forms, suggesting that man has often
tried to explain how organic evolution has
taken place by an appeal to the method which
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6 THEORY OF EVOLUTION
he believes he makes use of himself in the in-
organic world.
What has the evolution of the stars, of the
horse and of himian inventions in common?
Only this, that in each case from a simple be-
ginning through a series of changes something
more complex, or at least different, has come
into being. To lump all these kinds of changes
into one and call them evolution is no
more than asserting that you believe in con-
secutive series of events (which is history)
causally connected (which is science) ; that is,
that you believe in history and that you believe
in science. But let us not forget that we may
have complete faith in both without thereby
offering any explanation of either. It is the
business of science to find out specifically what
kinds of events were involved when the stars
evolved in the sky, when the horse evolved on
the earth, and the steam engine was evolved
from the mind of man.
Is it not rather an empty generalization to
say that any kind of change is a process of evo-
lution? At most it means little more than that
you want to intimate that miraculous interven-
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THEORY OF EVOLUTION 7
tion is not necessary to account for such kinds
of histories.
We are concerned here more particularly
with the biologists' ideas of evolution. My in-
tention is to review the evidence on which the
old theory rested its case, in the light of some
of the newer evidence of recent years.
Four great branches of study have fur-
nished the evidence of organic evolution. They
are:
Comparative anatomy.
Embryology.
Paleontology.
Experimental Breeding or Genetics.
The Evidence from Comparative Anatomy
When we study animals and plants we find
that they can be arranged in groups according
to their resemblances. This is the basis of com-
parative anatomy, which is only an accurate
study of facts that are superficially obvious to
everyone.
The groups are based not on a single differ-
ence, but on a very large nimiber of resem-
blances. Let us take for example the group of
vertebrates.
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8 THEORY OF EVOLUTION
The hand and the arm of man are similar to
the hand and arm of the ape. We find the
same plan in the forefoot of the rat, the ele-
phant, the horse and the opossum. We can
identify the same parts in the forefoot of the
lizard, the frog (fig. 3), and even, though less
Fig. 3. Limb skeletons of extinct and living animals, show-
ing the homologous bones: 1, salamander; 2, frog; 3, turtle;
4, Aetosaurus; 5, Pleisiosaurus ; 6, Ichthyosaurus; 7, Meso-
saurus; 8, duck. (After Jordan and Kellogg.)
certainly, in the pectoral fins of fishes. Com-
parison does not end here. We find similarities
in the skull and back bones of these same ani-
mals; in the brain; in the digestive system; in
the heart and blood vessels ; in the muscles.
Each of these systems is very complex, but
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THEORY OF EVOLUTION 9
the same general arrangement is fouiid in all.
Anyone familiar with the evidence will, I think,
probably reach the conclusion either that these
animals have been created on some precon-
ceived plan, or else that they have some other
bond that unites them; for we find it difficult
to beUeve that such complex, yet similar things
could have arisen independently. But we try
to convince our students of the truth of the
theory of evolution not so much by calling their
attention to this relation as by tracing each
organ from a simple to a complex structure.
I have never known such a course to fail in
its intention. In fact, I know that the student
often becomes so thoroughly convinced that
he resents any such attempt as that which I
am about to make to point out that the evidence
for his conviction is not above criticism.
Because we can often arrange the series of
structures in a line extending from the very
simple to the more complex, we are apt to be-
come unduly impressed by this fact and con-
clude that if we found the complete series we
should find all the intermediate steps and that
they have arisen in the order of their complex-
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10
THEORY OF EVOLUTION
ity. This conclusion is not necessarily correct.
Let me give some examples that have come
under my own observation. We have bred for
five years the wild fruit fly Drosophila ampelo-
phila (fig. 4) and we have found over a hun-
FiG. 4. Drosophila ampelophila. a, Female and b, male.
dred and twenty-five new types that breed true.
Each has arisen independently and suddenly.
Every part of the body has been affected by
one or another of these mutations. For in-
stance many different kinds of changes have
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THEORY OF EVOLUTION
11
taken place in the wings and several of these
involve the size of the wings. If we arrange
the latter arbitrarily in the order of their size
there will be an almost complete series begin-
ning with the normal wings and ending with
those of apterous flies. Several of these types
are represented in figure 5. The order in which
these mutations occurred bears no relation to
Fig. 5. Mutants of Drosophila ampelophila arranged in
order of size of wings: (a) cut; (b) beaded; (c) stumpy;
(d) another individual of stumpy; (f) vestigial (g) apterous.
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12 THEORY OF EVOLUTION
their size; each originated independently from
the wild type.
The wings of the wild fly are straight (fig.
4). Several types have arisen in which the
wings are bent upwards and in the most ex-
treme type the wings are curled over the back,
as seen in figure 55 (g) , yet there is no histori-
cal connection between these stages.
Mutations have occurred involving the pig-
mentation of the body and wings. The head
and thorax of the wild Drosophila ampelophila
are grayish yellow, the abdomen is banded with
yellow and black, and the wings are gray.
There have appeared in our cultures several
kinds of darker types ranging to almost black
flies (fig. 20) and to lighter types that are
quite yellow. If put in line a series may be
made from the darkest flies at one end to the
light yellow flies at the other. These types, with
the fluctuations that occur within each type,
furnish a complete series of gradations; yet
historically they have arisen independently of
each other.
Many changes in eye color have appeared.
As many as thirty or more races differing in eye
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THEORY OF EVOLUTION 13
color are now maintained in our cultures.
Some of them are so similar that they can
scarcely be separated from each other. It is
easily possible beginning with the darkest eye
color, sepia, which is deep brown, to pick out a
perfectly graded series ending with pure white
eyes. But such a serial arrangement would
give a totally false idea of the way the different
types have arisen; and any conclusion based
on the existence of such a series might very
well be entirely erroneous, for the fact that such
a series exists bears no relation to the order in
which its members have appeared.
Suppose that evolution "in the open" had
taken place in the same way, by means of dis-
continuous variation. What value then would
the evidence from comparative anatomy have
in so far as it is based on a continuous series of
variants of any organ?
No one familiar with the entire evidence will
doubt for a moment that these 125 races of
Drosophila ampelophila belong to the same
species and have had a common origin, for while
they may differ mainly in one thing they are
extremely alike in a hundred other things, and
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14 THEORY OF EVOLUTION
in the general relation of the parts to each
other.
It is in this sense that the evidence from
comparative anatomy can be used I think as
an argument for evolution. It is the resem-
blances that the animals or plants in any group
have in common that is the basis for such a con-
clusion; it is not because we can arrange in a
continuous series any particular variations. In
other words, our inference concerning the com-
mon descent of two or more species is based on
the totality of such resemblances that still re-
main in large part after each change has taken
place. In this sense the argument from com-
parative anatomy, while not a demonstration,
carries with it, I think, a high degree of
probability.
The Evidence from Embryology
In passing from the egg to the adult the
individual goes through a series of changes.
In the course of this development we see not
only the beginnings of the organs that gradu-
ally enlarge and change into those of the adult
animal, but also see that organs appear and
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THEORY OF EVOLUTION 15
later disappear before the adult stage is
reached. We find, moreover, that the young
sometimes resemble in a most striking way the
adult stage of groups that we place lower in the
scale of evolution.
Many years before Darwin advanced his
theory of evolution through natural selection,
the resemblance of the young of higher ani-
mals to the adults of lower animals had at-
tracted the attention of zoologists and various
views, often very naive, had been advanced
to account for the resemblance. Among
these speculations there was one practically
identical with that adopted by Darwin and the
post-Darwinians, namely that the higher ani-
mals repeat in their development the adfult
stages of lower animals. Later this view be-
came one of the cornerstones of the theory of
organic evolution. It reached its climax in the
writings of Haeckel, and I think I may add
without exaggeration that for twenty-five years
it furnished the chief inspiration of the school
of descriptive embryology. Today it is taught
in practically all textbooks of biology. Haeckel
called this interpretation the Biogenetic Law.
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16 THEORY OF EVOLUTION
It was recognized, of course, that many em-
bryonic stages could not possibly represent
ancestral animals. A young fish with a huge
j^olk sac attached (fig. 6) could scarcely ever
have led a happy, free life as an adult individ-
^ — •>*•*
Fig. 6. Young trout (Trutta fario) six days after hatching.
(After Ziegler.)
ual. Such stages were interpreted, however,
as embryonic additions to the original ancestral
type. The embryo had done something on its
own account.
In some animals the young have structures
that attach them to the mother, as does the
placenta of the mammals. In other cases the
young develop membranes about themselves —
like the amnion of the chick (fig. 7) and
mammal — that would have shut off an adult
animal from all intercourse with the outside
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THEORY OF EVOLUTION
17
world. Hundreds of such embryonic adapta-
tions are known to embryologists. These were
explained as adaptations and as falsifications
of the ancestral records.
At the end of the last century Weismann in-
FiG. 7. Diagram of chick showing relations of amnion,
allantois and yolk. (After Lillie.)
jected a new idea into our views concerning
the origin of variations. He urged that varia-
tions are germinal, i.e. they first appear in the
egg and the sperm as changes that later bring
about modifications in the individual. The
idea has been fruitful and is generally accepted
by most biologists today. It means that the
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18 THEORY OF EVOLUTION
offspring of a pair of animals are not affected
by the structure or the activities of their par-
ents, but the germ plasm is the unmodified
stream from which both the parent and the
young have arisen. Hence their resemblance.
Now, it has been found that a variation arising
in the germ plasm, no matter what its cause,
may affect any stage in the development of the
next individuals that arise from it. There is
no reason to suppose that such a change pro-
duces a new character that always sticks it-
self, as it were, on to the end of the old series.
This idea of germinal variation therefore car-
ried with it the death of the older conception
of evolution by superposition.
In more recent times another idea has be-
come current, mainly due to the work of
Bateson and of de Vries — ^the idea that varia-
tions are discontinuous. Such a conception
does not fall easily into line with the statement
of the biogenetic "law" ; for actual experience
with discontinuous variation has taught us that
new characters that arise do not add themselves
to the end of the line of already existing char-
acters but if they affect the adult characters
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THEORY OF EVOLUTION
19
they change them without, as it were, passing
through and beyond them.
I venture to think that these new ideas and
Fig. 8. Diagram of head of chick A and B, showing gill
slits, and aortic arches; and head of fish C showing aortic
arches. (After Hesse.)
this new evidence have played havoc with the
biogenetic "law". Nevertheless, there is an in-
terpretation of the facts that is entirely com-
Fio. 9. Human embryo showing gill slits and aortic arches.
(After His; from Marshall.)
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20
THEORY OF EVOLUTION
patible with the theory of evolution. Let me
illustrate this by an example.
The embryos of the chick (fig. 8) and of
man (fig. 9) possess at an early stage in their
development gill-slits on the sides of the neck
like those of fishes. No one familiar with the
Fig. 10. Young fish, dorsal view, and side view, showing
gill slits. (After Kopsch.)
relations of the parts will for a moment doubt
that the gill slits of these embryos and of the
fish represent the same structures. When we
look further into the matter we find that young
fish also possess gill slits (fig. 10 and 11) — even
in young stages in their development. Is it not
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THEORY OF EVOLUTION 21
then more probable that the mammal and
bird possess this stage in their development
simply because it has never been lost? Is not
this a more reasonable view than to suppose
that the gill slits of the embryos of the higher
forms represent the adult gill slits of the fish
Fig. 11. Side views of head of embryo sharks, showing gill
slits. (After Sedgwick.)
that in some mysterious way have been pushed
back into the embryo of the bird?
I could give many similar examples. All
can be interpreted as embryonic survivals
rather than as phyletic contractions. Not one
of them calls for the latter interpretation.
The study of the cleavage pattern of the
segmenting egg furnishes the most convincing
evidence that a different explanation from the
one stated in the biogenetic law is the more
probable explanation.
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22 THEORY OF EVOLUTION
It has been found that the cleavage pattern
has the same general arrangement in the early
stages of flat worms, annehds and molluscs
(fig. 12). Obviously these stages have never
Fig. 12. Cleavage stages of four types of eggs, showing the
origin of the mesenchyme cells (stippled) and mesoderm cells
(darker); a, Planarian; b. Annelid (Podarke) ; c, Mollusc
(Crepidula), c, Mollusc (Unio).
been adult ancestors, and obviously if their
lesemblance has any meaning at all, it is that
each group has retained the same general plan
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THEORY OF EVOLUTION 23
of cleavage, possessed by their common
ancestor.
Accepting this view, let us ask, does the evi-
dence from embryology favor the theory of
evolution? I think that it does very strongly.
The embryos of the mammal, bird, and lizard
have gill slits today because gill slits were pres-
ent in the embryos of their ancestors. There is
no other view that explains so well their pres-
ence in the higher forms.
Perhaps someone will say, Well! is not this
all that we have contended for! Have you
not reached the old conclusion in a roundabout
way? I think not. To my mind there is a
wide difference between the old statement that
the higher animals living today have the origi-
nal adult stages telescoped into their embryos,
and the statement that the resemblance be-
tween certain characters in the embryos of
higher animals and corresponding stages in the
embryos of lower animals is most plausibly ex-
plained by the assimaption that they have
descended from the same ancestors, and that
their common structures are embryonic sur-
vivals.
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24 THEORY OF EVOLUTION
The Evidence from Paleontology
The direct evidence furnished by fossil re-
mains is by all odds the strongest evidence that
we have in favor of organic evolution. Paleon-
tology holds the incomparable position of being
able to point directly to the evidence showing
that the animals and plants living in past times
are connected with those living at the present
time, often through an unbroken series of
stages. Paleontology has triumphed over the
weakness of the evidence, which Darwin ad-
mitted was serious, by filling in many of the
missing links.
Paleontology has been criticised on the
ground that she cannot pretend to show the
actual ancestors of living forms because, if in
the past genera and species were as abundant
and as diverse as we find them at present, it is
very improbable that the bones of any individual
that happened to be preserved are the bones of
just that species that took part in the evolution.
Paleontologists will freely admit that in many
cases this is probably true, but even then the
evidence is, I think, still just as valuable and
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THEORY OF EVOLUTION 25
in exactly the same sense as is the evidence from
comparative anatomy. It suffices to know that
there lived in the past a particular "group" of
animals that had many points in common with
those that preceded them and with those that
came later. Whether these are the actual an-
cestors or not does not so much matter, for the
view that from such a group of species the later
species have been derived is far more probable
than any other view that has been proposed.
With this unrivalled material and splendid
series of gradations, paleontology has con-
structed many stages in the past history of the
globe. But paleontologists have sometimes
gone beyond this descriptive phase of the sub-
ject and have attempted to formulate the
"causes", "laws" and "principles" that have led
to the development of their series. It has even
been claimed that paleontologists are in an in-
comparably better position than zoologists to
discover such principles, because they know
both the beginning and the end of the evolu-
tionary series. The retort is obvious. In his
sweeping and poetic vision the paleontologist
may fail completely to find out the nature of
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26 THEORY OF EVOLUTION
the pigments that have gone into the painting
of his picture, and he may confuse a familiarity
with the different views he has enjoyed of the
canvas with a knowledge of how the painting
is being done.
My good friend the paleontologist is in
greater danger than he reaUzes, when he leaves
descriptions and attempts explanation. He
has no way to check up his specidations and it is
notorious that the himian mind without con-
trol has a bad habit of wandering.
When the modem student of variation and
heredity — ^the geneticist — looks over the differ-
ent "continuous" series, from which certain
"laws" and "principles" have been deduced, he
is struck by two facts: that the gaps, in some
cases, are enormous as compared with the single
changes with which he is familiar, and (what is
more important) that they involve nimierous
parts in many ways. The geneticist says to the
paleontologist, since you do not know, arid
from the nature of your case can never know,
whether your differences are due to one change
or to a thousand, you can not with certainty
tell us anything about the hereditary units
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THEORY OF EVOLUTION 27
which have made the process of evolution possi-
ble. And without this knowledge there can be
no understanding of the causes of evolution.
THE FOUR GREAT HISTORICAL
SPECULATIONS
Looking backward over the history of the
evolution theory we recognize that during the
hundred and odd years that have elapsed since
Buffon, there have been four main lines of
speculation concerning evolution. We might
call them the four great cosmogonies or the
four modern epics of evolution.
The Environment
Geoffroy St. Hilaire
About the beginning of the last century
Geoffroy St. Hilaire, protege, and in some
respects a disciple of Buff on, was interested as
to how Uving species are related to the animals
and plfuits that had preceded them. He was
f amihar with the kind of change that takes
place in the embryo if it is put into new or
changed surroundings, and from this knowl-
edge he concluded that as the surface of the
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^8 THEORY OF EVOLUTION
earth slowly changed — as the carbon dioxide
contents in the air altered — as land appeared —
and as marine animals left the water to inhabit
it, they or their embryos responded to the new
conditions and those that responded favorably
gave rise to new creations. As the environ-
ment changed the fauna and flora changed —
change for change. Here we have a picture of
progressive evolution that carries with it an
idea of mechanical necessity. If there is any-
thing mystical or even improbable in St. Hi-
liare's argument it does not appear on the sm*-
face; for he did not assimie that the response to
the new environment was always a favorable
one or, as we say, an adaptation. He expressly
stated that if the response was unfavorable the
individual or the race died out. He assumed
that sometimes the change might be favorable,
i.e., that certain species, entire groups, would
respond in a direction favorable to their exist-
ence in a new environment and these would
come to inherit the earth. In this sense he an-
ticipated certain phases of the natural selection
theory of Darwin, but only in part; for his
picture is not one of strife within and without
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THEORY OF EVOLUTION 29
the species, but rather the escape of the species
from the old into a new world.
If then we recognize the intimate bond in
chemical constitution of living things and of the
world in which they develop, what is there im-
probable in St. Hilaire's hypothesis? Why, in
a word is not more credit given to St. Hilaire
in modern evolutionary thought? The reasons
are to be found, I think, first, in that the evi-
dence to which he appealed was meagre and
inconclusive; and, second, in that much of his
special evidence does not seem to us to be ap-
plicable. For example the monstrous forms
that development often assumes in a strange
environment, and with which every embryolo-
gist is only too familiar, rarely if ever furnish
combinations, as he supposed, that are capable
of living. On the contrary, they lead rather to
the final catastrophe of the organism. And
lastly, St. Hilaire's appeal to sudden and great
transformations, such as a crocodile's egg
hatching into a bird, has exposed his view to too
easy ridicule.
But when all is said, St. Hilaire's conception
of evolution contains elements that form the
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30 THEORY OF EVOLUTION
background of our thinking to-day, for taken
broadly, the interaction between the organism
and its environment was a mechanistic concep-
tion of evolution even though the details of the
theory were inadequate to establish his con-
tention.
In our own time the French metaphysician
Bergson in his Evolution Creatrice has pro-
posed in mystical form a thought that has at
least a superficial resemblance to St. Hilaire's
conception. The response of living things is no
longer hit in one species and miss in another;
it is precise, exact; yet not mechanical in the
sense at least in which we usually employ the
word mechanical. For Bergson claims that
the one chief feature of living material is that
it responds favorably to the situation in which
it finds itself; at least so far as lies within the
possible physical limitations of its organization.
Evolution has followed no preordained plan;
it has had no creator ; it has brought about its
own creation by responding adaptively to each
situation as it arose.
But note : the man of science believes that the
organism responds today as it does, because at
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THEORY OF EVOLUTION 31
present it has a chemical and physical constitu-
tion that gives this response. We find a speci-
fic chemical composition and generally a specific
physical structure already existing. We have
no reason to suppose that such particular reac-
tions would take place until a specific chemical
configuration had been acquired. Where did
this constitution come from? This is the ques-
tion that the scientist asks himself. I suppose
Bergson would have to reply that it came into
existence at the moment that the first specific
stimulus was applied. But if this is the answer
we have passed at once from the realm of obser-
vation to the realm of fancy — ^to a realm that
is foreign to our experience ; for such a view as-
sumes that chemical and physical reactions are
guided by the needs of the organism when the
reactions take place inside living beings.
Use and Disuse
From Lamarck to Weismann
The second of the four great historical ex-
planations appeals to a change not immedi-
ately connected with the outer world, but to
one within the organism itself.
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S2 THEORY OF EVOLUTION
Practice makes perfect is a familiar adage.
Not only in human aflpairs do we find that a
part through use becomes a better tool for
performing its task, and through disuse de-
generates; but in the field of animal behavior
we find that many of the most essential types
of behavior have been learned through repeated
associations formed by contact with the outside.
It was not so long ago that we were taught
that the instincts of animals are the inherited
experience of their ancestors — lapsed intelli-
gence was the current phrase.
Lamarck's name is always associated with
the application of the theory of the inheritance
of acquired characters. Darwin fully en-
dorsed this view and made use of it as an expla-
nation in all of his writings about animals.
Today the theory has few followers amongst
trained investigators, but it still has a popular
vogue that is widespread and vociferous.
To Weismann more than to any other single
individual should be ascribed the disfavor into
which this view has fallen. In a series of bril-
liant essays he laid bare the inadequacy of the
supposed evidence on which the inheritance of
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THEORY OF EVOLUTION 33
acquired characters rested. Your neighbor's
cat, for instance, has a short tail, and it is said
that it had its tail pinched oflp by a closing door.
In its litter of kittens one or more is found
without a tail. Your neighbor believes that
here is a case of cause and effect. He may even
have known that the mother and grandmother
of the cat had natural tails. But it has been
found that short tail is a dominant character;
therefore, until we know who was the father of
the short-tailed kittens the accident to its
mother and the normal condition of her mater-
nal ancestry is not to the point.
Weismann appealed to common sense. He
made few experiments to disprove Lamarck's
hypothesis. True, he cut oflp the tails of some
mice for a few generations but got no tailless
oflpspring and while he gives no exact measure-
ments with coefficients of error he did not ob-
serve that the tails of the descendants had
shortened one whit. The combs of fighting
cocks and the tails of certain breeds of sheep
have been cropped for many generations and
the practice continues today, because their tails
are still long. While in Lamarck's time there
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34 THEORY OF EVOLUTION
was no evidence opposed to his ingenious the-
ory, based as it was on an appeal to the ac-
knowledged facts of improvement that take
place in the organs of an individual through
their own functioning (a fact that is as obvious
and remarkable today as in the time of La-
marck), yet now there is evidence as to
whether the effects of use and disuse are inher-
ited, and this evidence is not in accord with
Lamarck's doctrine.
THE UNFOLDING PRINCIPLE
Ndgeli and Bateson
I have ventured to put down as one of the
four great historical explanations, under the
heading of the unfolding principle, a conception
that has taken protean forms. At one extreme
it is little more than a mystic sentiment to the
effect that evolution is the result of an inner
driving force or principle which goes under
many names such as Bildungstrieb, nisus for-
mativus, vital force, and orthogenesis. Evolu-
tionary thought is replete with variants of this
idea, often naively expressed, sometimes uncon-
sciously implied. Evolution once meant, in
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THEORY OF EVOLUTION 35
fact, an unfolding of what pre-existed in the
eggy and the term still carries with it some-
thing of its original significance.
Nageli's speculation written several years
after Darwin's "Origin of Species" may be
taken as a typical case. Nageli thought that
there exists in living material an innate power
to grow and expand. He vehemently pro-
tested that he meant only a mechanical prin-
ciple but as he failed to refer such a principle
to any properties of matter known to physicists
and chemists his view seems still a mysterious
affirmation, a:s difficult to understand as the
facts themselves which it purports to explain.
Nageli compared the process of evolution
to the growth of a tree, whose ultimate twigs
represent the living world of species. Natural
selection plays only the role of the gardener
who prunes the tree into this or that shape but
who has himself produced nothing. As an
imaginative figure of speech Nageli's compari-
son of the tree might even today seem to hold
if we substituted "mutations" for "growth",
but although we know so little about what
causes mutations there is no reason for suppos-
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36 THEORY OF EVOLUTION
ing them to be due to an inner impulse, and
hence they furnish no justification for such a
hypothesis.
In his recent presidential address before the
British Association Bateson has inverted this
idea. I suspect that his effort was intended as
little more than a tour de force. He claims
for it no more than that it is a possible line of
speculation. Perhaps he thought the time had
come to give a shock to our too confident views
concerning evolution. Be this as it may, he
has invented a striking paradox. Evolution
has taken place through the steady loss of in-
hibiting factors. Living matter was stopped
down, so to speak, at the beginning of the
world. As the stops are lost, new things
emerge. Living matter has changed only in
that it has become simpler.
Natural Selection
Darwin
Of the four great historical speculations
about evolution, the doctrine of Natural Selec-
tion of Darwin and Wallace has met with the
most widespread acceptance. In the last lec-
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THEORY OF EVOLUTION 37
ture I intend to examine this theory critically.
Here we are concerned only with its broadest
aspects.
Darwin appealed to chance variations as
supplying evolution with the material on which
natural selection works. If we accept, for the
moment, this statement as the cardinal doctrine
of natural selection it may appear that evolu-
tion is due, (1) not to an orderly response of
the organism to its environment, (2) not
in the main to the ajctivities of the animal
through the use or disuse of its parts, (3) not
to any inniate principle of living material itself,
and (4) above all not to purpose either from
within or from without. Darwin made quite
clear what he meant by chance. By chance he
did not mean that the variations were not
causal. On the contrary he taught that in
Science we mean by chance only that the par-
ticular combination of causes that bring about
a variation are not known. They are accidents,
it is true, but they are causal accidents.
In his famous book on "Animals and Plants
under Domestication", Darwin dwells at great
length on the nature of the conditions that
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38 THEORY OF EVOLUTION
bring about variations. If his views seem to us
today at times vague, at times problematical,
and often without a secure basis, nevertheless
we find in every instance, that Darwin was
searching for the physical causes of variation.
He brought, in consequence, conviction to
many minds that there are abundant indica-
tions, even if certain proof is lacking, that the
causes of variation are to be found in natural
processes.
Today the belief that evolution takes place
by means of natural processes is generally ac-
cepted. It does not seem probable that we
shall ever again have to renew the old contest
between evolution and special creation.
But this is not enough. We can never re-
main satisfied with a negative conclusion of this
kind. We must find out what natural causes
bring about variations in animals and plants;
and we must also find out what kinds of varia-
tions are inherited, and how they are inherited.
If the circimistantial evidence for organic evo-
lution, furnished by comparative anatomy,
embryology and paleontology is cogent, we
should be able to observe evolution going on at
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THEORY OF EVOLUTION 39
the present time, i.e. we should be able to
observe the occurrence of variations and their
transmission. This has actually been done by
the geneticist in the study of mutations and
Mendelian heredity, as the succeeding lectures
will show.
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CHAPTER II
THE BEARING OF MENDEL'S DISCOVERY
ON THE ORIGIN OF HEREDITARY
CHARACTERS
Between the years 1857 and 1868 Gregor
Mendel, Augustinian monk, studied the hered-
ity of certain characters of the common edible
pea, in the garden of the monastery at Briinn.
In his account of his work written in 1868,
he said:
"It requires indeed some courage to undertake a
labor of such a far-reaching extent ; it appears, how-
ever, to be the only right way by which we can finally
reach the solution of a question the importance of
which cannot be over-estimated in connection with
the history of the evolution of organic forms."
He tells us also why he selected peas for his
work:
"The selection of the plant group which shall serve
for experiments of this kind must be made with all
possible care if it be desired to avoid from the outset
every risk of questionable results."
"The experimental plants must necessarily
40
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THEORY OF EVOLUTION 41
1. Possess constant differentiating characters.
2, The hybrids of such plants must, during the
flowering period, be protected from the influence of all
foreign pollen, or be easily capable of such protec-
tion."
Why do biologists throughout the world to-
day agree that Mendel's discovery is one of
first rank?
A great deal might be said in this connec-
tion. What is essential may be said in a few
words. Biology had been, and is still, largely
a descriptive and speculative science. Mendel
showed by eaoperimental proof that heredity
could he eooplained by a simple mechanism.
His discovery has been exceedingly fruitful.
Science begins with naive, often mystic con-
ceptions of its problems. It reaches its goal
whenever it can replace its early guessing by-
verifiable hypotheses and predictable results.
This is what Mendel's law did for heredity.
Mendel's First Discovery — Segregation
Let us turn to the demonstration of his first
law — ^the law of segregation. The first case I
choose is not the one given by Mendel but one
worked out later by Correns. If the common
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n THEORY OF EVOLUTION
garden plant called four o'clock (Mirabilis jal-
apa) with red flowers is crossed to one having
white flowers, the offspring are pink (fig. 13) .
The hybrid, then, is intermediate in the color of
Fig. 13. Diagram illustrating a cross between a red (dark)
and a white variety of four o'clock (Mirabilis jalapa).
its flowers between the two parents. If these
hybrids are inbred the offspring are white, pink
and red, in the proportion of 1:2:1. All of
these had the same ancestry, yet they are of
three different kinds. If we did not know their
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THEORY OF EVOLUTION 43
history it would be quite impossible to state
what the ancestry of the white or of the red
had been, for they might just as well have come
from pure white and pure red ancestors respec-
tively as to have emerged from the pink hy-
brids. Moreover, when we test them we find
that they are as pure as are white or red flower-
ing plants that have had all white or all red
flowering ancestors.
Mendel's Law explains the results of this
cross as shown in figure 14.
The egg cell from the white parent carries
the factor for white, the pollen cell from the red
parent carries the factor for red. The hybrid
formed by their union carries both factors.
The result of their combined action is to pro-
duce flowers intermediate in color.
When the hybrids mature and their germ
cells (eggs or pollen) ripen, each carries only
one of these factors, either the red or the white,
but not both. In other words, the two factors
that have been brought together in the hybrid
separate in its germ cells. Half of the egg
cells are white bearing, half red bearing. Half
of the pollen cells are white bearing, half red
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44 THEORY OF EVOLUTION
bearing. Chance combinations at fertilization
give the three classes of individuals of the sec-
ond generation.
The white flowering plants should forever
breed true, as in fact they do. The red flowering
Q PARENTS I
1
Fig. 14. Diagram illustrating the history of the factors in
the germ cells of the cross shown in Fig. 13.
plants also breed true. The pink flowering
plants, having the same composition as the hy-
brids of the first generation, should give the
same kind of result. They do, indeed, give this
result i.e. one white to two pink to one red
flowered oflPspring,
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THEORY OF EVOLUTION
45
Another case of the same kind is known to
breeders of poultry. One of the most beautiful
of the domesticated breeds is known as the An-
(
Fig. 15. Diagram illustrating a cross between special races
of white and black fowls, producing the blue (here gray)
Andalusian.
dalusian. It is a slate blue bird shading into
blue-black on the neck and back. Breeders
know that these blue birds do not breed true
but produce white, black, and blue offspring.
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46
THEORY OF EVOLUTION
The explanation of the failure to produce a
pure race of Andalusians is that they are like
the pink flowers of the four o'clock, i.e., they are
a hybrid type formed by the meeting of the
white and the black germ cells. If the whites
produced by the Andalusians are bred to the
Fig. 16. Diagram showing history of germ cells of cross of
Fig. 15. The larger circles indicate the color of the birds;
their enclosed small circles the nature of the factors in the
germ cells of such birds.
blacks (both being pure strains), all the off-
spring will be blue (fig. 15) ; if these blues are
inbred they will give 1 white, to 2 blues, to 1
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THEORY OF EVOLUTION
47
black. In other words, the factor for white and
the factor for black separate in the germ cells
of the hybrid Andalusian birds (fig. 16) .
The third case is Mendel's classical case
of yellow and green peas (fig. 17) . He crossed
a plant belonging to a race having yellow peas
with one having green peas. The hybrid plants
had yellow seeds. These hybrids inbred gave
three yellows to one green. The explanation
Fig. 17. Diagram of Mendel's cross between yellow (dom-
inant) and green (recessive) peas.
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48 THEORY OF EVOLUTION
(fig. 18) is the same in principle as in the pre-
ceding cases. The only difference between
them is that the hybrid which contains both the
yellow and the green factors is in appearance
Fig. 18. Diagram illustrating the history of the factors in
the cross shown in Fig. 17.
not intermediate, but like the yellow parent
stock. Yellow is said therefore to be dominant
and green to be recessive.
Another example where one of the con-
trasted characters is dominant is shown by the
cross of Drosophila with vestigial wings to
the wild type with long wings (fig. 19). The
Fi flies have long wings not differing from
those of the wild fly, so far as can be observed.
When two such flies are inbred there result
three long to one vestigial.
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THEORY OF EVOLUTION
49
ammn)
ammiD
Vestigial
Long
c
(amiiD)
Gametes orF,
CUD . dTTTTTni) Eggs
t X '
3
(nTTTTTn )
CZZ)
(nTTTTTn)
(nnmn)
annnn)
Fig. 19. Diagram illustrating a cross between a fly (Dro-
sophila ampelophila) with long wings and a mutant fly with
vestigial wings.
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50 THEORY OF EVOLUTION
The question as to whether a given character
is dominant or recessive is a matter of no theo-
retical importance for the principle of segre-
y, gation, although from the notoriety given to it
one might easily be misled into the erroneous
-I supposition that it was the discovery of this re-
• • lation that is Mendel's crowning achievement.
'^ * Let me illustrate by an example in which the
' hybrid standing between two types overlaps
5 them both. There are two mutant races in our
cultures of the fruit fly Drosophila that have
dark body color, one called sooty, another which
is even blacker, called ebony (fig. 20). Sooty
crossed to ebony gives offspring that are inter-
mediate in color. Some of them are so much
like sooty that they cannot be distinguished
from sooty. At the other extreme some of the
hybrids are as dark as the lightest of the ebony
flies. If these hybrids are inbred there is a con-
tinuous series of individuals, sooties, interme-
diates and ebonies. Which color here shall we
call the dominant? If the ebony, then in the
second generation we coimt three ebonies to
one sooty, putting the hybrids with the ebonies.
If the dominant is the sooty then we coimt three
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THEORY OF EVOLUTION
51
sooties to one ebony, putting the hybrids with
the sooties. The important fact to find out is
whether there actually exist three classes in the
second generation. This can be ascertained
CO
Ct3
X
s ^
o >
r X
C "^ :^
£ o o
-<> <
~ m
2:^
Fig. 20. Cross between two allelomorphic races of Dro-
sophila, sooty and ebony, that give a completely graded series
in F^.
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52 THEORY OF EVOLUTION
even when, as in this ease, there is a perfectly
graded series from one end to the other, by
testing out individually enough of the flies to
show that one-fourth of them never produce
any descendants but ebonies, one-fourth never
any but sooties, and one-half of them give rise
to both ebony and sooty.
Mendel's Second Discovery — Independent
Assortment
Besides his discovery that there are pairs of
characters that disjoin, as it were, in the germ
cells of the hybrid (law of segregation) Men-
del made a second discovery which also has
f ar-reachiiig consequences. The following case
illustrates Mendel's second law.
If a pea that is yellow and roimd is crossed
to one that is green and wrinkled (fig. 21), all
of the offspring are yellow and round. Inbred,
these give 9 yellow round, 3 green round, 3
yellow wrinkled, 1 green wrinkled. All the
yellows taken together are to the green as 3: 1.
All the round taken together are to the wrin-
kled as three to one ; but some of the yellows are
now wrinkled and some of the green are now
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THEORY OF EVOLUTION 53
round. There has been a recombination of char-
acters, while at the same time the results, for
each pair of characters taken separately, are in
DCO • <0 ©
ooo • @
Fig. 21. Cross between yellow-round and green-wrinkled
peas, giving the 9: 3: 3: 1 ratio in F^.
accord with Mendel's Law of Segregation,
(fig. 22). The second law of Mendel may be
called the law of independent assortment of
different character pairs.
We can, as it were, take the characters of
one organism and recombine them with those
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54
THEORY OF EVOLUTION
of a different organism. We can explain this
result as due to the assortment of factors for
these characters in the germ cells according to
a definite law.
As a second illustration let me take the clas-
fj PARENTS ^n
YR GW
' 'YR YW GR GW) v-
'' YR YW GR GW; ,-
YR
YR
YW
YR
Y R
YW
YR
GR
yr"
GW
o
YW
GR
O
GR
YR
GR
;
\ YW
GW
©
O
Fig. 22. Diagram to show the history of the factor pairs
yellow-green and round-wrinkled of the cross in Fig. 21.
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THEORY OF EVOLUTION 55
sic case of the combs of fowls. If a bird with a
rose comb is bred to one with a pea comb (fig*
23), the offspring have a comb different from
either. It is called a walnut comb. If two
such individuals are bred they give 9 walnut.
*
f
#\
f\ ¥\ it\ it'
f\
Fig. 23. Cross between pea ana rose combed fowls. (Charts
of Baur and Goldschmidt.)
3 rose, 3 pea, 1 single. This proportion shows
that the grandparental types differed in re-
spect to two pairs of characters.
A fourth case is shown in the fruit fly, where
an ebony fly with long wings is mated to a grey
fly with vestigial wings (fig. 24). The off-
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56
THEORY OF EVOLUTION
(nmnDx
(flniiiDx
Vestigial Gray
^^^ p
Long Ebony
Fig. 24>. Cross between long ebony and gray vestigial flies.
spring are gray with long wings. If these are
inbred they give 9 gray long, 3 gray vestigial,
3 ebony long, 1 ebony vestigial (figs. 24 and
25).
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THEORY OF EVOLUTION 57
The possibility of interchanging characters
might be illustrated over and over again. It is
true not only when two pairs of characters are
involved, but when three, four, or more enter
the cross.
10 i
10 i
10
in
10!
01
Hi
01
10
10
00
II
01
II i
oil
01 !
Fig. 25. Diagram to show the history of the factors in the
cross shown in Fig. 24.
It is as though we took individuals apart
and put together parts of two, three or more
individuals by substituting one part for another.
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58 THEORY OF EVOLUTION
Not only has this power to make whatever
combinations we choose great practical impor-
tance, it has even greater theoretical signifi-
cance; for, it follows that the individual is not
in itself the imit in heredity, but that within the
germ-cells there exist smaller units concerned
with the transmission of characters.
The older mystical statement of the individ-
ual as a imit in heredity has no longer any in-
terest in the light of these discoveries, except
as a past phase of biological history. We see,
too, more clearly that the sorting out of factors
in the germ plasm is a very different process
from the influence of these factors on the devel-
opment of the organism. There is today no
excuse for confusing these two problems.
If mechanistic principles apply also to em-
bryonic development then the course of devel-
opment is capable of being stated as a series
of chemico-physical reactions and the 'Hndi-
viduaV is merely a term to express the sum
total of such reactions and should not be in-
terpreted as something different from or more
than these reactions. So long as so little is
known of the actual processes involved in devel-
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THEORY OF EVOLUTION 59
opment the use of the term "individuality",
while giving the appearance of profundity, in
reaUty often serves merely to cover ignorance
and to make a mystery out of a mechanism.
The Characters of Wild Animals and
Plants Follow the Same Laws of In-
heritance AS DO THE Characters of
Domesticated Animals and Plants.
Darwin based many of his conclusions con-
cerning variation and heredity on the evidence
derived from the garden and from the stock
farm. Here he was handicapped to some ex-
tent, for he had at times to rely on informa-
tion much of which was uncritical, and some of
which was worthless.
Today we are at least better informed on
two important points; one concerning the
kinds of variations that furnish to the cultiva-
tor the materials for his selection; the other
concerning the modes of inheritance of these
variations. We know now that new charac-
ters are continually appearing in domesti-
cated as well as in wild animals and plants,
that these characters are often sharply marked
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60 THEORY OF EVOLUTION
off from the original characters, and whether
the differences are great or whether they are
small they are transmitted alike according to
Mendel's law.
Many of the characteristics of om* domesti-
cated animals and cultivated plants originated
long ago, and only here and there have the
records of their first appearance been pre-
served. In only a few instances are these rec-
ords clear and definite, while the complete
history of any large group of our domesticated
products is unknown to us.
Within the last five or six years, however,
from a common wild species of fly, the fruit
fly, Drosophila ampelophila, which we have
brought into the laboratory, have arisen over a
hundred and twenty-five new types whose
origin is completely known. Let me call at-
tention to a few of the more interesting of
these types and their modes of inheritance,
comparing them with wild types in order to
show that the kinds of inheritance found in do-
mesticated races occur also in wild types. The
results will show beyond dispute that the char-
acters of wild types are inherited in precisely
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THEORY OF EVOLUTION 61
the same way as are the characters of the mu-
tant types — a fact that is not generally appre-
ciated except by students of genetics, although
it is of the most far-reaching significance for
the theory of evolution.
A mutant appeared in which the eye color
of the female was different from that of the
male. The eye color of the mutant female is
a dark eosin color, that of the male yellowish
eosin. From the beginning this difference was
as marked as it is to-day. Breeding experi-
ments show that eosin eye color differs from
the red color of the eye of the wild fly by a
single mutant factor. Here then at a single
step a type appeared that was sexually
dimorphic.
Zoologists know that sexual dimorphism is
not uncommon in wild species of animals, and
Darwin proposed the theory of sexual selec-
tion to account for the difference between the
sexes. He assumed that the male preferred
certain kinds of females differing from himself
in a particular character, and thus in time
through sexual selection, the sexes came to
differ from each other.
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62 THEORY OF EVOLUTION
In the case of eosin eye color no such process
as that postulated by Darwin to account for
the differences between the sexes was involved;
nn
wt
L
Fig. 26. Clover butterfly (Colias philodice) with two types
of females, above; and one type of male, below.
for the single mutation that brought about the
change also brought in the dimorphism with it.
In recent years zoologists have carefully
studied several cases in which two types of fe-
male are found in the same species. In the
common clover butterfly, there is a yellow and
a white type of female, while the male is yellow
(fig. 26). It has been shown that a single
factor difference determines whether the female
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THEORY OF EVOLUTION 63
is yellow or white. The inheritance is, accord-
ing to Gerould, strictly Mendelian.
In Papilio tumus there exist, in the southern
states, two kinds of females, one yellow like
the male, one black (fig. 27). The evidence
here is not so certain, but it seems probable that
Fig. 27. Papilio turnus with two types of females above and
one type of male below.
a single factor difference determines whether
the female shall be yellow or black.
Finally in Papilio polytes of Ceylon and
India three different types of females appear.
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64 THEORY OF EVOLUTION
(fig. 28 to right) only one of which is like the
male. Here the analysis of the breeding data
shows the possibility of explaining this case
Fig. 2S. Papilio polytes, with three types of female to right
and one type of male above to left.
as due to two pairs Mendelian factors which
give in combination the three types of female.
Taking these cases together, they furnish
a much simpler explanation than the one pro-
posed by Darwin. They show also that char-
acters like these shown by wild species may
follow Mendel's law.
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THEORY OF EVOLUTION 65
There has appeared in our cultures a fly in
which the third division of the thorax with its
appendages has changed into a segment Uke
the second (fig. 29). It is smaller than the
Fig. 29. Mutant race of fruit fly with intercalated dupli-
cate mesothorax on dorsal side.
normal mesothorax and its wings are imper-
fectly developed, but the bristles on the upper
surface may have the typical arrangement of
the normal mesothorax. The mutant shows
how great a change may result from a single
factor difference.
A factor that causes duplication in the legs
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66
THEORY OF EVOLUTION
has also been found. Here the interesting
fact was discovered (Hoge) that duplication
takes place only in the cold. At ordinary tem-
peratures the legs are normal.
In contrast to the last case, where a charac-
ter is doubled, is the next one in which the eyes
are lost (fig. 30) . This change also took place
at a single step. All the flies of this stock
Fig. 30. Mutant race of fruit fly, called eyeless ; a, a' normal
eye.
however, cannot be said to be eyeless, since
many of them show pieces of the eye — ^indeed
the variation is so wide that the eye may even
appear like a normal eye unless carefully ex-
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THEORY OF EVOLUTION 67
amined. Formerly we were taught that eye-
less animals arose in caves. This case shows
that they may also arise suddenly in glass milk
bottles, by a change in a single factor.
I may recall in this connection that wingless
flies (fig. 5 f ) also arose in our cultures by a
single mutation. We used to be told that
wingless insects occurred on desert islands be-
cause those insects that had the best developed
wings had been blown out to sea. Whether
this is true or not, I will not pretend to say,
but at any rate wingless insects may also arise,
not through a slow process of elimination, but
at a single step.
The preceding examples have all related
Fig. 31. Mutant race of fruit fly called bar to the right
(normal to the left). The eye is a narrow vertical bar, the
outline of the original eye is indicated.
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68 THEORY OF EVOLUTION
to recessive characters. The next one is
dominant.
A single male appeared with a narrow verti-
cal red bar (fig. 31) instead of the broad red
oval eye. Bred to wild females the new char-
acter was found to dominate, at least to the
extent that the eyes of all its offspring were
narrower than the normal eye, although not so
narrow as the eye of the pure stock. Around
Fig. 32. Mutant race of fruit fly, called speck. There is a
minute black speck at base of wing.
the bar there is a wide border that cor-
responds to the region occupied by the rest of
the eye of the wild fly. It lacks however the
elements of the eye. It is therefore to be
looked upon as a rudimentary organ, which is,
so to speak, a by-product of the dominant
mutation.
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THEORY OF EVOLUTION
69
The preceding eases have all involved rather
great changes in some one organ of the body.
The following three cases involve slight
changes, and yet follow the same laws of inheri-
tance as do the larger changes.
At the base of the wings a minute black
speck appeared (fig. 32). It was fomid to
be a Mendehan character. In another case the
spines on the thorax became forked or kinky
Fig. 33. Mutant race of fruit fly called club. The wings
often remain unexpanded and two bristles present in wild
fly (b) are absent on side of thorax (c).
(fig. 53b). This stock breeds true, and the
character is inherited in strictly Mendelian
fashion.
In a certain stock a number of flies appeared
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70 THEORY OF EVOLUTION
in which the wing pads did not expand (fig.
33). It was found that this peculiarity is
shown in only about twenty per cent of the in-
dividuals supposed to inherit it. Later it was
found that this stock lacked two bristles on the
sides of the thorax. By means of this knowl-
edge the heredity of the character was easily
determined. It appears that while the expan-
sion of the wing pads fails to occur once in five
times — probably because it is an environmental
effect peculiar to this stock, — yet the minute
difference of the presence or absence of the two
lateral bristles is a constant feature of the flies
that carry this particular factor.
In the preceding cases I have spoken as
though a factor influenced only one part of the
body. It would have been more accurate to
have stated that the chief effect of the factor
was observed in a particular part of the body.
Most students of genetics realize that a factor
difference usually affects more than a single
character. For example, a mutant stock called
rudimentary wings has as its principle character-
istic very short wings (fig. 34) . But the factor
for rudimentary wings also produces other ef-
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THEORY OF EVOLUTION 71
fects as well. The females are almost com-
pletely sterile, while the males are fertile. The
viability of the stock is poor. When flies with
rudimentary wings are put into competition
Fig. 34. Mutant race of fruit fly, called rudimentary.
with wild flies relatively few of the rudimentary
flies come through, especially if the culture is
crowded. The hind legs are also shortened.
All of these effects are the results of a single
factor-difference.
One may venture the guess that some of the
specific and varietal differences that are char-
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72 THEORY OF EVOLUTION
acteristic of wild types and which at the same
time appear to have no survival value, are only
by-products of factors whose most important
effect is on another part of the organism where
their influence is of vital importance.
It is well known that systematists make use
of characters that are constant for groups of
species, but which do not appear in themselves
to have an adaptive significance. If we may
suppose that the constancy of such characters
may be only an index of the presence of a
factor whose chief influence is in some other
direction or directions, some physiological in-
fluence, for example, we can give at least a
reasonable explanation of the constancy of
such characters.
I am incUned to think that an overstatement
to the effect that each factor may affect the en-
tire body, is less likely to do harm than to state
that each factor affects only a particular char-
acter. The reckless use of the phrase "unit
character" has done much to mislead the unini-
tiated as to the effects that a single change in
the germ plasm may produce on the organism.
Fortunately, the expression "unit character"
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THEORY OF EVOLUTION 73
is being less used by those students of genetics
who are more careful in regard to the implica-
tions of their terminology.
There is a class of cases of inheritance, due to
the XY chromosomes, that is called sex linked
inheritance. It is shown both by mutant char-
acters and characters of wild species.
For instance, white eye color in Drosoph-
ila shows sex linked inheritance. If a white
eyed male is mated to a wild red eyed female
(fig. 35) all the offspring have red eyes. If
these are inbred, there are three red to one
white eyed offspring, but white eyes occur only
in the males. The grandfather has transmitted
his peculiarity to half of his grandsons, but to
none of his granddaughters.
The reciprocal cross (fig. 36) is also inter-
esting. If a white eyed female is bred to a red
eyed male, all of the daughters have red eyes
and all of the sons have white eyes. We call
this criss-cross inheritance. If these offspring
are inbred, they produce equal nimibers of
red eyed and white eyed females and equal
numbers of red eyed and white eyed males. The
ratio is 1: 1: 1: 1, or ignoring sex, 2 reds to
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74
THEORY OF EVOLUTION
2 whites, and not the usual 3 : 1 Mendelian
ratio. Yet, as will be shown later, the result
is in entire accord with Mendel's principle of
segregation.
Fig. 35. Diagram showing a cross between a white eyed male
and a red eyed female of the fruit fly. Sex linked inheritance.
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THEORY OF EVOLUTION
75
Fig. 36. Diagram illustrating a cross between a red eyed
male and white eyed female of the fruit fly (reciprocal cross
(of that shown in Fig. 35).
It has been shown by Sturtevant that in a
wild species of Drosophila, viz., D. repleta,
two varieties of individuals exist, in one of
which the thorax has large splotches and in the
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76 THEORY OF EVOLUTION
other type smaller splotches (fig. 37). The
factors that differentiate these varieties are sex
linked.
Certain types of color blindness (fig. 38)
and certain other abnormal conditions in man
such as haemophilia, are transmitted as sex
linked characters.
Fig. 37. Two types of markings on thorax of Drosophila
repleta, both found "wild". They show sex linked inheritance.
In domestic fowls sex linked inheritance has
been found as the characteristic method of
transmission for at least as many as six char-
acters, but here the relation of the sexes is in
a sense reversed. For instance, if a black
Langshan hen is crossed to a barred Plymouth
Rock cock (fig. 39), the offspring are all
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THEORY OF EVOLUTION 77
"^ W
^^
xxxo
Mil-
^^^XX XX X© X0
Fig. 38, A. Diagram illustrating inheritance of color blind-
ness in man; the iris of the color-blind eye is here black.
XO XX
XX X0
Fig. 38, B. Reciprocal of cross in Fig. 38 a.
barred. If these are inbred half of the daugh-
ters are black and half are barred all of the
sons are barred. The grandmother has trans-
mitted her color to half of her granddaughters
but to none of her grandsons.
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78
THEORY OF EVOLUTION
In the reciprocal cross (fig. 40) black cock
by barred hen, the daughters are black and the
sons barred — criss-cross inheritance. These
Fig. 39. Sex-linked inheritance in domesticated birds shown
here in a cross between barred Plymouth Rock male and black
Langshan female.
inbred give black hens and black cocks, barred
hens and barred cocks.
There is a case comparable to this found in
a wild species of moth. Abraxas grossulariata.
A wild variation of this type is lighter in color
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THEORY OF EVOLUTION
79
and is known as A. lacticolor. When these two
types are crossed they exhibit exactly the same
type of heredity as does the black-barred com-
FiG. 40. Reciprocal of Fig. 39.
bination in the domestic fowl. As shown in
figure 41, lacticolor female bred to grossula-
riata male gives grossulariata sons and daugh-
ters. These inbred give grossulariata males
and females and lacticolor females. Recipro-
cally lacticolor male by grossulariata female,
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80
THEORY OF EVOLUTION
(fig. 42) gives lacticolor daughters and gros-
sulariata sons and these inbred give grossu-
lariata males and females and lacticolor males
and females.
GROSSULARIATA 9
o®
cccsO (D
(D.(D
(E) apcrai
Fio. 41. Sex-linked inheritance in the wild moth. Abraxas
grossulariata (darker) and A. lacticolor.
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THEORY OF EVOLUTION
81
It has been found that there may be even
more than two factors that show Mendelian
segregation when brought together in pairs.
For example, in the southern States there are
several races of the grouse locust (Paratettix)
ucncoLORg
C^OSSULARIATA d"
Fio. 42. Reciprocal of Fig. 4?1.
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THEORY OF EVOLUTION
that differ from each other markedly in color
patterns (fig. 43). When any two individuals
of these races are crossed they give, as Nabours
has shown, in F2 a Mendelian ratio of 1: 2: 1.
It is obvious, therefore, that there are here at
least nine characters, any two of which be-
have as a Mendelian pair. These races have
n \
Fig. 43. Four wild types of Paratettix in upper line with
three hybrids below, (After Naboure.)
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THEORY OF EVOLUTION 83
arisen in nature and differ definitely and strik-
ingly from each other, yet any two differ by
only one factor difference.
Similar relations have been found in a num-
ber of domesticated races. In mice there is a
quadruple system represented by the gray house
mouse, the white bellied, the yellow and the
black mouse (fig. 44). In rabbits there is
<^ d.
Fig. 44. Diagram illustrating four allelomorphs in mice, viz.
gray bellied gray (wild type) (above, to left) ; white bellied
gray (above, to right) ; yellow (below, to right) ; and black
(below, to left).
probably a triple system, that includes the al-
bino, the Himalayan, and the black races. In
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84 THEORY OF EVOLUTION
the silkworm moth there have been described
four types of larvae, distinguished by different
color markings, that form a system of quad-
ruple allelomorphs. In Drosophila there is a
quintuple system of factors in the sex chromo-
some represented by eye colors, a triple system
of body colors, and a triple system of factors for
eye colors in the third chromosome.
Mutation and Evolution
What bearing has the appearance of these
new types of Drosophila on the theory of evo-
lution may be asked. The objection has been
raised in fact that in the breeding work with
Drosophila we are dealing with artificial and
unnatural conditions. It has been more than
implied that results obtained from the breed-
ing pen, the seed pan, the flower pot and the
milk bottle do not apply to evolution in the
''open", nature "at large" or to "wild" types.
To be consistent, this same objection should be
extended to the use of the spectroscope in the
study of the evolution of the stars, to the use
of the test tube and the balance by the chemist,
of the galvanometer by the physicist. All these
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THEORY OF EVOLUTION 85
are unnatural instruments used to torture Na-
ture's secrets from her. I venture to think that
the real antithesis is not between imnatural
and natural treatment of Nature, but rather
between controlled or verifiable data on the one
hand, and imrestrained generalization on the
other.
If a systematist were asked whether these
new races of Drosophila are comparable to
wild species, he would not hesitate for a mo-
ment. He would call them all one species. If
he were asked why, he would say, I think,
"These races differ only in one or two striking
points, while in a hundred other respects they
are identical even to the minutest details." He
would add, that as large a group of wild spe-
cies of flies would show on the whole the reverse
relations, viz.^ they would differ in nearly every
detail and be identical in only a few points.
In all this I entirely agree with the systematist,
for I do not think such a group of types dif-
fering by one character each, is comparable to
most wild groups of species because the differ-
ence between wild species is due to a large num-
ber of such single differences. The characters
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86 THEORY OF EVOLUTION
that have been accumulated in wild species are
of significance in the maintenance of the species,
or at least we are led to infer that even though
the visible character that we attend to may not
itself be important, one at least of the other
effects of the factors that represent these char-
acters is significant. It is, of course, hardly to
be expected that any random change in as com-
plex a mechanism as an insect would improve
the mechanism, and as a matter of fact it is
doubtful whether any of the mutant types so
far discovered are better adapted to those con-
ditions to which a fly of this structure and hab-
its is already adjusted. But this is beside the
mark, for modern genetics shows very posi-
tively that adaptive characters are inherited in
exactly the same way as are those that are not
adaptive ; and I have already pointed out that
we cannot study a single mutant factor without
at the same time studying one of the factors
responsible for normal characters, for the two
together constitute the Mendelian pair.
And, finally, I want to urge on your atten-
tion a question that we are to consider in more
detail in the last lecture. Evolution of wild
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THEORY OF EVOLUTION 87
species appears to have taken place by modify-
ing and improving bit by bit the structures
and habits that the animal or plant already
possessed. We have seen that there are thirty
mutant factors at least that have an influence
on eye color, and it is probable that there are
at least as many normal factors that are in-
volved in the production of the red eye of the
wild fly.
Evolution from this point of view has con-
sisted largely in introducing new factors that
influence characters already present in the ani-
mal or plant.
Such a view gives us a somewhat different
picture of the process of evolution from the old
idea of a ferocious struggle between the indi-
viduals of a species with the survival of the
fittest and the annihilation of the less fit. Evo-
lution assimies a more peaceful aspect. New
and advantageous characters survive by incor-
porating themselves into the race, improving it
and opening to it new opportunities. In other
words, the emphasis may be placed less on the
competition between the individuals of a spe-
cies (because the destruction of the less fit does
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88 THEORY OF EVOLUTION
not in itself lead to anything that is new) than
on the appearance of new characters and modi-
fications of old characters that become incor-
porated in the species, for on these depends the
evolution of the race.
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CHAPTER III
THE FACTORIAL THEORY OF HEREDITY AND
THE COMPOSITION OF THE GERM PLASM
The discovery that Mendel made with edible
peas concerning heredity has been found to ap-
ply everywhere throughout the plant and
animal kingdoms — to flowering plants, to in-
sects, snails, Crustacea, fishes, amphibians,
birds, and mammals (including man) .
There must be something that these widely
separated groups of plants and animals have
in common — some simple mechanism per-
haps — to give such definite and orderly series
of results. There is, in fact, a mechanism,
possessed alike by animals and plants, that ful-
fills every requirement of Mendel's principles.
The Cellular Basis of Organic Evolution
AND Heredity
In order to appreciate the full force of the
evidence, let me first pass rapidly in review a
89
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90 THEORY OF EVOLUTION
few familiar, historical facts, that preceded the
discovery of the mechanism in question.
Throughout the greater part of the last cen-
tury, while students of evolution and of hered-
FiG. 45. Typical cell showing the cell wall, the protoplasm
(with its contained materials) ; the nucleus with its contained
chromatin and nuclear sap. (After Dahlgren.)
ity were engaged in what I may call the more
general, or, shall I say, the grosser aspects of
the subject, there existed another group of stu-
dents who were engaged in working out the
minute structure of the material basis of the
living organism. They found that organs such
as the brain, the heart, the liver, the limgs, the
kidneys, etc., are not themselves the units of
structure, but that all these organs can be re-
duced to a simpler imit that repeats itself a
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THEORY OF EVOLUTION 91
thousand-fold in every organ. We call this
unit a cell (fig. 45).
The egg is a cell, and the spermatozoon is a
cell. The act of fertilization is the union of two
cells (fig. 47, upper figure). Simple as the
process of fertilization appears to us today, its
discovery swept aside a vast amount of mys-
tical speculation concerning the role of the
male and of the female in the act of procreation.
Within the cell a new microcosm was re-
vealed. Every cell was found to contain a
spherical body called the nucleus (fig. 46a).
Within the nucleus is a network of fibres, a
sap fills the interstices of the network. The net-
work resolves itself into a definite number of
threads at each division of the cell (fig.
46 b-e). These threads we call chromosomes.
Each species of animals and plants possesses
a characteristic number of these threads which
have a definite size and sometimes a specific
shape and even characteristic granules at dif-
ferent levels. Beyond this point our strongest
microscopes fail to penetrate. Observation has
reached, for the time being, its limit.
The story is taken up at this point by a new
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THEORY OF EVOLUTION
u
c
1
^^3k;
1
msi'-^:^.
d e
Fig. 46. A series of cells in process of cell division. The
chromosomes are the black threads and rods. (After
Dahlgren.)
set of students who have worked in an entirely
different field. Certain observations and ex-
periments that we have not time to consider
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THEORY OF EVOLUTION 93
now, led a number of biologists to conclude that
the chromosomes are the bearers of the heredi-
tary units. If so, there should be many such
units carried by each chromosome, for the Jium-
ber of chromosomes is hmited while the number
of independently inherited characters is large.
In Drosophila it has been demonstrated not only
that there are exactly as many groups of char-
acters that are inherited together as there are
pairs of chromosomes, but even that it is possi-
ble to locate one of these groups in a particular
chromosome and to state the relative position
there of the factors for the characters. If the
validity of this evidence is accepted, the study
of the cell leads us finally in a mechanical, but
not in a chemical sense, to the ultimate units
about which the whole process of the transmis-
sion of the hereditary factors centers.
But before plunging into this somewhat tech-
nical matter (that is difficult only because it is
unfamiliar), certain facts which are familiar
for the most part should be recalled, because
on these turns the whole of the subsequent
story.
The thousands of cells that make up the cell-
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94
THEORY OF EVOLUTION
state that we call an animal of plant come from
the fertilized egg. An hour or two after fer-
tilization the egg divides into two cells (fig.
47). Then each half divides again. Each
Od
.*•*
Fig. 47. An egg, and the division of the egg — the so-called
process of cleavage. (After Selenka.)
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THEORY OF EVOLUTION
95
quarter next divides. The process continues
until a large number of cells is formed and out
of these organs mould themselves.
At every division of the cell the chromosomes
also divide. Half of these have come from the
mother, half from the father. Every cell con-
FiG. 48. Section of the egg of the beetle, Calligrapha, show-
ing the pigment at one end where the germ cells will later
develop as shown in the other two figures. (After Hegner.)
tains, therefore, the sum total of all the chro-
mosomes, and if these are the bearers of the
hereditary qualities, every cell in the body.
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96 THEORY OF EVOLUTION
whatever its function, has a common inheri-
tance.
At an early stage in the development of the
animal certain cells are set apart to form the
organs of reproduction. In some animals these
cells can be identified early in the cleavage
(fig. 48).
The reproductive cells are at first like all the
other cells in the body in that they contain a
full complement of chromosomes, half paternal
and half maternal in origin (fig. 49). They
divide as do the other cells of the body for a
long time (fig. 49, upper row). At each
division each chromosome splits lengthwise and
its halves migrate to opposite poles of the spin-
dle (fig. 49 c).
But there comes a time when a new process
appears in the germ cells (fig 49 e-h). It is
essentially the same in the egg and in the sperm
cells. The discovery of this process we owe to
the laborious researches of many workers in
many countries. The list of their names is
long, and I shall not even attempt to repeat it.
The chromosomes come together in pairs (fig.
49 e) . Each maternal chromosome mates with
a paternal chromosome of the same kind.
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THEORY OF EVOLUTION
97
Fig. 49. In the upper row of the diagram a typical process
of nuclear division, such as takes place in the early germ cells
or in the body cells. In the lower row the separation of the
chromosomes that have paired. This sort of separation takes
place at one of the two reduction divisions.
Then follow two rapid divisions (fig. 49 f,
g and 50 and 51) . At one of the divisions the
double chromosomes separate so that each re-
sulting cell comes to contain some maternal and
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98
THEORY OF EVOLUTION
Fig. 50. The two maturation divisions of the sperm cell.
Four sperms result, each with half (haploid) the full number
(diploid) of chromosomes.
some paternal chromosomes, i.e. one or the
other member of each pair. At the other di-
vision each chromosome simply sphts as in
ordinary cell division.
The upshot of the process is that the ripe
eggs (fig. 51) and the ripe spermatozoa (fig.
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THEORY OF EVOLUTION
99
Fig. 51. The two maturation divisions of the egg. The divi-
sions are unequal, so that two small polar bodies are formed
one of these subsequently divides. The three polar bodies
and the egg are comparable to the four sperms.
50) come to contain only half the total num-
ber of chromosomes.
When the eggs are fertilized the whole num-
ber of chromosomes is restored again.
The Mechanism or Mendelian Heredity
Discovered in the Behavior or
THE Chromosomes
If the factors in heredity are carried in the
chromosomes and if the chromosomes are defin-
ite structures, we should anticipate that there
should be as many groups of characters as
there are kinds of chromosomes. In only one
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-^.^
100 THEORY OF EVOLUTION
case has a sufficient number of characters been
studied to show whether there is any corre-
spondence between the number of hereditary
groups of characters and the number of chro-
mosomes. In the fruit fly, Drosophila ampelo-
phila, we have found about 125 characters that
are inherited in a perfectly definite way. On
the opposite page is a hst of some of them.
It will be observed in this list that the charac-
ters are arranged in four groups, Groups I,
II, III and IV. Three of these groups are
9* ^ equally large or nearly so ; Group IV contains
only two characters. The characters are put into
these groups because in heredity the members
of each group tend to be inherited together,
i.e., if two or more enter the cross together they
tend to remain together through subsequent
generations. On the other hand, any member
of one group is inherited entirely independently
of any member of the other groups ; in the same
way as Mendel's yellow-green pair of charac-
ters is inherited independently of the round-
wrinkled pair.
If the factors for these characters are car-
ried by the chromosomes, then we should ex-
»
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THEORY OF EVOLUTION
101
Group I
Group II
Group III
Group IV
Abnormal
Antlered
Band
Bent
Bar
Apterous
Beaded
Eyeless
Bifid
Arc
Cream III
Bow
Balloon
Deformed
O)
Cherry
Black
Dwarf
X
C X
C3 >
Chrome
BUstered
Ebony
Cleft
Comma
Giant
Club
Confluent
Kidney
^ 5
Depressed
Cream II
Low crossing
over
r- "^ »
Dot
Curved
Maroon
Eosin
Dachs
Peach
< > <
Facet
Extra vein
Pink
Forked
Fringed
Rough
Furrowed
Jaunty
Safranin
Fused
Limited
Sepia
r rn
Green
Little crossover Sooty
Jaunty
Morula
Spineless
Lemon
Olive
Spread
S :i
Lethals, 13
Plexus
Trident
o
Miniature
Purple
Truncate intensifier
Notch
Speck
Whitehead
lieduplicated
Strap
White ocelli
Ruby
Streak
Rudimentary
Trefoil
Sable
Truncate
Shifted
Vestigial
Short
Skee
Spoon
Spot
Tan
Truncate intensifier
Vermilion
White
Yellow
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lOa THEORY OF EVOLUTION
pect that those factors that are carried by the
same chromosome would be inherited together,
provided the chromosomes are definite struc-
tures in the cell.
In the chromosome group of Drosophila,
(fig. 52) there are four pairs of chromosomes,
three of nearly the same size and one much
smaller. Not only is there agreement between
JJK <«4
A /f
Fig. 52. Chromosomes (diploid) of D. ampelophila. The
sex chromosomes are XX in the female and XY in the male.
There are three other pairs of chromosomes.
the number of hereditary groups and the num-
ber of the chromosomes, but even the size rela-
tions are the same, for there are three great
groups of characters and three pairs of large
chromosomes, and one small group of charac-
ters and one pair of small chromosomes.
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THEORY OF EVOLUTION 103
The Four Great Linkage Groups of
Drosophila ampelophila
The following description of the characters
of the wild fly may be useful in connection with
the account of the modifications of these char-
acters that appear in the mutants.
The head and thorax of the wild fly are gray-
ish-yellow, the abdomen is banded with alter-
nate stripes of yellow and black. In the male,
(fig. 4 to right), there are three narrow bands
and a black tip. In the female there are five
black bands (fig. 4 to left) . The wings are gray
with a surface texture of such a kind that at cer-
tain angles they are iridescent. The eyes are a
deep, solid, brick-red. The minute hairs that
cover the body have a very definite arrange-
ment that is most obvious on the head and
thorax. There is a definite number of larger
hairs called bristles or chaetae which have a
characteristic position and are used for diagnos-
tic purposes in classifying the species. On the
foreleg of the male there is a comb-like organ
formed by a row of bristles ; it is absent in the
female. The comb is a secondary sexual char-
acter, and it is, so far as known, functionless.
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104 THEORY OF EVOLUTION
Some of the characters of the mutant types
are shown in figures 53, 54, 55, 56. The draw-
ing of a single fly is often used here to illustrate
more than one character. This is done to econ-
omize space, but of course there would be no
difficulty in actually bringing together in the
same individual any two or more characters be-
longing to the same group (or to different
groups). Without colored figures it is not
possible to show many of the most striking dif-
ferences of these mutant races; at most dark
and light coloring can be indicated by the
shading of the body, wings, or eyes.
Group I
In the six flies drawn in figure 53 there are
shown five different wing characters. The
first of these types (a) is called cut, because the
ends of the wings look as though they had been
cut to a point. The antennae are displaced
downward and appressed and their bristle-like
aristae are crumpled.
The second figure (b) represents a fly with a
notch in the ends of the wings. This charac-
ter is dominant, but the same factor that pro-
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THEORY OF EVOLUTION 105
Fig. 53. Group I. (See text)
duces the notch in the wings is also a recessive
lethal factor ; because of this latter effect of the
character no males of this race exist, and the
females of the race are never pure but hy-
brid. Every female with notch wings bred
to a wild male, will produce in equal num-
bers notch winged daughters and daugh-
ters with normal wings. There will be half as
many sons as daughters. The explanation of
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106 THEORY OF EVOLUTION
this peculiar result is quite simple. Every notch
winged female has one X chromosome that
carries the factor for notch and one X chromo-
some that is "normal". Daughters receiving
the former chromosomes are notched because
the factor for notch is dominant, but they are
not killed since the lethal effect of the notch
factor is recessive to the normal allelomorph
carried by the other chromosome that the
daughters get from their father. This
normal factor is recessive for notch but domi-
nant for life. This same figure (b) is used
here to show three other sex linked characters.
The spines on the thorax are twisted or kinky,
which is due to a factor called "forked". The
effect is best seen on the thorax, but all spines
on the body are similarly modified; even the
minute hairs are also affected. Ruby eye color
might be here represented — if the eyes in the
figure were colored. The lighter color of the
body and antennae is intended to indicate that
the character tan is also present. The hght
color of the antennae is the most certain way of
identifying tan. The tan flies are interesting
because they have lost the positive heliotropism
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THEORY OF EVOLUTION 107
that is so marked a feature in the behavior of
D. ampelophila. As this peculiarity of the tan
flies is inherited like all the other sex linked
characters, it follows that when a tan female is
bred to a wild male all the sons inherit the re-
cessive tan color and indifference to light, while
the daughters show the dominant sex linked
character of their father, i.e., they are "gray",
and go to the light. Hence when such a brood
is disturbed the females fly to the light, but the
males remain behind.
One of the first mutants that appeared in
D. ampelophila was called rudimentary on ac-
count of the condition of the wings (c) . The
same mutation has appeared independently
several times. In the drawing (c) the dark
body color is intended to indicate "sable" and
the lighter color of the eyes is intended to indi-
cate eosin. This eye color, which is an allelo-
morph of white, is also interesting because in
the female the color is deeper than in the male.
In other cases of sex linked factors the char-
acter is the same in the two sexes.
In the fourth figure (d) the third and fourth
longitudinal veins of the wing are fused into
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108 THEORY OF EVOLUTION
one vein from the base of the wing to the level
of the first cross- vein and in addition converge
and meet near their outer ends. The shape of
the eye is represented in the figure as different
from the normal, due to another factor called
"bar". This is a dominant character, the hybrid
condition being also narrow, but not so narrow
as the pure type. Vermilion eye color might
also be here represented — due to a factor
that has appeared independently on several
occasions.
In the fifth figure (e) the wings are shorter
and more pointed than in the wild fly. This
character is called miniature. The light color
of the drawing may be taken to represent yel-
low body color, and the light color of the eye
white eye color.
In the last figure (f) the wings are repre-
sented as pads, essentially in the same condi-
tion that they are in when the fly emerges from
the pupa case. Not all the flies of this stock have
the wings in this condition ; some have fully ex-
panded wings that appear normal in all re-
spects. Nevertheless, about the same percen-
tage of offspring show the pads irrespective of
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THEORY OF EVOLUTION 109
whether the parents had pads or expanded
wings.
The flies of this stock show, however, another
character, which is a product of the same factor,
and which is constant, i.e., repeated in all in-
dividuals. The two bristles on the sides of
the thorax are constantly absent in this race.
The lighter color of the eye in the figure may
be taken to indicate buff — a faint yellowish
color. The factor for this eye color is another
allelomorph of white.
There are many other interesting characters
that belong to the first group, such as abnormal
abdomen, short legs, duplication of the legs,
etc. In fact, any part of the body may be af-
fected by a sex-linked factor.
Group II
In the first figure (a) of figure 54 that
contains members of Group II the wings are
almost entirely absent or "vestigial". This
condition arose at a single step and breeds
true, although it appears to be influenced to
some extent by temperature, also by modifiers
that sometimes appear in the stock. Purple
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110
THEORY OF EVOLUTION
^^/
Fig. 54. Group II. (See text.)
eye color belongs in Group II; it resembles
the color of the eye of the wild fly but is darker
and more translucent.
In the second figure (b) the wing is again
long and narrow and sometimes bent back on
itself, as shown here. In several respects the
wing resembles strap (d) but seems to be due
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THEORY OF EVOLUTION 111
to another factor, called antler, insufficiently
studied as yet.
In the third figure (c) the wings turn up at
the end. This is brought about by the presence
of the factor called jaunty.
In the fourth figure the wings are long and
narrow and several of the veins are unrepre-
sented. This character, "strap", is very varia-
ble and has not yet been thoroughly studied.
On the thorax there is a deep black mark called
trefoil. Even in the wild fly there is a three
pronged mark on the thorax present in many
individuals. Trefoil is a further development
and modification of this mark and is due to a
special factor.
In the fifth figure (e) the wings are arched.
The factor is called arc. The dark color of the
body, and especially of the wings, indicates the
factor for black.
The sixth figure (f) shows the wings
"curved" downwards. In addition there is
present a minute black speck at the base of
each wing, due to another factor called speck.
In the seventh figure (g) the wing is trun-
cate. Its end is obliquely squared instead of
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112 THEORY OF EVOLUTION
rounded; it may be longer than the body, or
shorter when other modifying factors are pres-
ent. The mutation that produces this type of
wing is of not infrequent occurrence. It has
been shown by MuUer and Altenburg that
there are at least two factors that modify this
character — the chief factor is present in the
second chromosome; alone it produces the
truncate wing in only a certain percentage of
cases, but when the modifiers are also present
about ninety percent of the individuals may
show the truncate condition of the wing. But
the presence of these factors makes the stock
very infertile, so that it is difficult to maintain.
In the eighth figure (h) the legs are short-
ened owing to the absence of a segment of the
tarsus. The stock is called dachs — a nickname
given to it because the short legs suggested the
dachshund.
Group III
In figure 55, (a), a mutant type called bi-
thorax is shown. The old metathorax is re-
placed by another mesothorax thrust in between
the normal mesothorax and the abdomen. It
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THEORY OF EVOLUTION
113
carries a pair of wings that do not completely
unfold. On this new mesothorax the character-
istic arrangement of the bristles is shown. Thus
at a single step a typical region of the body
has doubled. The character is recessive.
The size of the adult fly of D. ampelophila
Fig. 55. Group III. (See text.)
varies greatly according to the amount of
nourishment obtained by the larva. After the
fly emerges its size remains nearly constant,
as in many insects. Two races have, how-
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114 THEORY OF EVOLUTION
ever, been separated by Bridges that are dif-
ferent in size as a result of a genetic factor.
The first of these, called dwarf, is represented
by figure 55y (b).
The race is minute, although of course its
size is variable, depending on food and other
conditions. The same figure shows the pres-
ence of another factor, "sooty", that makes the
fly very dark. Maroon eye color might be
here represented, due to still another factor.
In the third figure (c) the other mutation in
size is shown. It is called "giant". The flies
are twice the size of wild flies. An eye color,
called peach, might here be represented. It is
an allelomorph of pink.
In the fourth figure (d) the mutant called
dichaete is shown. It is characterized by the ab-
sence of two of the bristles on the thorax.
Other bristles may also be absent, but not so
constantly as the two just mentioned. An-
other effect of the same factor is the spread-out
condition of the wings. The very dark eye
color in this figure may be taken to indicate
the presence of another factor, "sepia", which
causes the eyes to assume a brown color that
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THEORY OF EVOLUTION 115
becomes black with age. Most of the other
mutations in eye color that have occurred tend
to give a lighter color: this one, which is also
recessive, makes the eye darker.
In the fifth figure (e) the color of the dark-
est fly is due to a factor called ebony, which is
an allelomorph of sooty.
In the sixth figure (f ) the wings are beaded,
i.e., the margin is defective at intervals, giving
a beaded-like outline to the wings. This con-
dition is very variable and much affected by
other factors that influence the shape of the
wings. The lighter eye color of the drawing
may be taken to represent pink.
In the seventh figure (g) the wings are
curled up over the back. This is a recessive
character.
Group IV
Only two mutants have been obtained that do
not belong to any of the preceding groups;
these are put together in Group IV. It has
been shown that they are linked to each other
and the linkage is so close that it has thus far
been impossible to obtain the double recessive.
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116
THEORY OF EVOLUTION
One of these mutants, called "eyeless" (fig. 56,
(a, a^), is variable — the eyes are often entirely
absent or represented by one or more groups of
ommatidia. The outline of the original eye,
ir% ^
Fig. b^. Group IV. (See text.)
SO to speak, is strongly marked out and its
area might be called a rudimentary organ, if
such a statement has any meaning here.
The other figure (b) represents "bent", so
called from the shape of the wings. This mu-
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THEORY OF EVOLUTION 117
tant is likewise very variable, often indistin-
guishable from the wild type, yet when well
developed strikingly different from any other
mutant.
This brief account of a few of the mutant
races that can be most easily represented by
uncolored figures will serve to show how all
parts of the body may change, some of the
changes being so slight that they would be
overlooked except by an expert, others so great
that in the character affected the flies depart
far from the original species.
It is important to note that mutations in the
first chromosome are not limited to any part
of the body nor do they affect more frequently
a particular part. The same statement holds
equally for all of the other chromosomes. In
fact, since each factor may affect visibly sev-
eral parts of the body at the same time there
are no grounds for expecting any special rela-
tion between a given chromosome and special
regions of the body. It can not too insistently
be urged that when we say a character is the
product of a particular factor we mean no
more than that it is the most conspicuous effect
of the factor.
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118 THEORY OF EVOLUTION
If, then, as these and other results to be de-
scribed point to the chromosomes as the bear-
ers of the Mendelian factors, and if, as will be
shown presently, these factors have a definite
location in the chromosomes it is clear that the
location of the factors in the chromosomes bears
no spatial relation to the location of the parts
of the body to each other.
Localization of Factors in the
Chromosomes
The Evidence from Sex Linked Inheritance
When we follow the history of pairs of
chromosomes we find that their distribution in
successive generations is paralleled by the in-
heritance of Mendelian characters. This is best
shown in the sex chromosomes (fig. 57). In
the female there are two of these chromosomes
that we call the X chromosomes; in the male
there are also two but one differs from those of
the female in its shape, and in the fact that it
carries none of the normal allelomorphs of the
mutant factors. It is called the Y chromosome.
The course followed by the sex chromosomes
and that by the characters in the case of sex
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THEORY OF EVOLUTION 119
rEMALE MALE
88 M
i>
•Gamctcs
98 8ff
Fig. 57. Scheme of sex determination in Drosophila type.
Each mature egg contains one X, each mature sperm contains
one X, or a Y chromosome. Chance union of any egg with
any sperm will give either XX (female) or XY (male).
linked inheritance are shown in the next dia-
gram of Drosophila illustrating a cross between
a white eyed male and a red eyed female.
The first of these represents a cross between
a white eyed male and a red eyed female (fig.
58, top row). The X chromosome in the
male is represented by an open bar, the Y
chromosome is bent. In the female the two X
chromosomes are black. Each egg of such a
female will contain one "black" X after the
polar bodies have been thrown off. In the male
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120
THEORY OF EVOLUTION
III
Fig. 58. Cross between white eyed male of D. ampelophila
and red eyed female. The sex chromosomes are indicated by
the rods. A black rod indicates that the chromosome carries
the factor for red; the open chromosome the factor for white
eye color.
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THEORY OF EVOLUTION 121
there will be two classes of sperm — ^the female-
producing, carrying the (open) X, and the
male-producing, carrying the Y chromosome.
Any egg fertilized by an X bearing sperm will
produce a female that will have red eyes be-
cause the X (black) chromosome it gets from
the mother carries the dominant factor for red.
Any egg fertiUzed by a Y-bearing sperm will
produce a male that will also have red eyes be-
cause he gets his (black) X chromosome from
his mother.
When, then, these two Fi flies (second row)
are inbred the following combinations are ex-
pected. Each egg will contain a black X (red
eye producing) or a white X (white eye pro-
ducing) after the polar bodies have been ex-
truded. The male will produce two kinds of
sperms, of which the female producing will
contain a black X (red eye producing) . Since
any egg may by chance be fertilized by any
sperm there will result the four classes of indi-
viduals shown on the bottom row of the dia-
gram. All the females will have red eyes,
because irrespective of the two kinds of eggs
involved all the female-producing sperm carry
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122 THEORY OF EVOLUTION
00
'0
ilT
/ \ ^
I SPERM h(T
Fig. 59. Cross between red eyed male and white eyed fe-
male; reciprocal cross of Fig. 58.
Digitized by
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THEORY OF EVOLUTION 123
a black X. Half of the males have red eyes
because half of the eggs have had each a red-
producing X chromosome. The other half of
the males have white eyes, because the other
half of the eggs had each a white-producing X
chromosome. Other evidence has shown that
the Y chromosome of the male is indifferent, so
far as these Mendelian factors are concerned.
The reciprocal experiment is illustrated in
figure 59. A white eyed female is mated to a
red eyed male (top row) . All the mature eggs
of such a female contain one white-producing
X chromosome represented by the open bar
in the diagram. The red eyed male contains fe-
male-producing X-bearing sperm that carry
the factor for red eye color, and male-produc-
ing Y chromosomes. Any egg fertilized by an
X-bearing sperm will become a red eyed female
because the X chromosome that comes from the
father carries the dominant factor for red eye
color. Any egg fertilized by a Y-bearing
sperm will become a male with white eyes be-
cause the only X chromosome that the male
contains comes from his mother and is white
producing.
Digitized by
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1^4 THEORY OF EVOLUTION
When these two Fi flies are inbred (middle
row) the following combinations are expected.
Half the eggs will contain each a white pro-
ducing X chromosome and half red producing.
The female-producing sperms will each con-
tain a white X and the male-producing sperms
will each contain an indifferent Y chromosome.
Chance meetings of egg and sperm will give the
four F2 classes (bottom row). These consist
of white eyed and red eyed females and white
eyed and red eyed males. The ratio here is
1:1 and not three to one (3:1) as in other
Mendelian cases. But Mendel's law of segre-
gation is not transgressed, as the preceding
analysis has shown; for, the chromosomes have
followed strictly the course laid down on Men-
del's principle for the distribution of factors.
The peculiar result in this case is due to the
fact that the Fi male gets his single factor for
eye color from his mother only and it is linked
to or contained in a body (the X chromosome)
that is involved in producing the females, while
the mate of this body — the Y chromosome — is
indifferent with regard to these factors, yet
active as a mate to X in synapsis.
Digitizechby
Googk
THEORY OF EVOLUTION 125
In man there are several characters that show
exactly this same kind of inheritance. Color
blindness, or at least certain kinds of color
blindness, appear to follow the same scheme. A
color blind father transmits through his daugh-
ters his peculiarity to half of his grandsons,
but to none of his grand-daughters (fig. 38 A) .
Diploid Nuclei
Gametes
XX XO
/\ /\
X X X
\ \ y 1
Fertilization
\ X /
Zygotes XX X6
Fig. 60. Diagram of sex determination in type with XX
female and XO male (after Wilson).
The result is the same as in the case of the white
eyed male of Drosophila. Color blind women
are rather unusual, which is expected from the
method of inheritance of this character, but in
the few known cases where such color blind
women have married normal husbands the sons
have inherited the peculiarity from the mother
(fig. 38B) . Here again the result is the same
as for the similar combination in Drosophila.
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126 THEORY OF EVOLUTION
In man the sex formula appears to be XX
for the female and XO for the male (fig. 60) ,
and since the relation is essentially the same as
that in Drosophila the chromosome explanation
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B
c
F
Fig. 61. Spermatogenesis in man. There are 47 chromo-
somes (diploid) in the male. After reduction half of the
sperm carry 24 chromosomes (one of which is X) and half
carry 23 chromosomes (no X).
is the same. According to von Winiwarter
there are 48 chromosomes in the female and 47
in the male (fig. 61). After the extrusion of
the polar bodies there are 24 chromosomes in the
^g^. In the male at one of the two maturation
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THEORY OF EVOLUTION 127
divisions the X chromosome passes to one pole
midivided (fig. 61, C). In consequence there
are two classes of sperms in man; female pro-
ducing containing 24 chromosomes, and male
producing containing 23 chromosomes. If the
factor for color blindness is carried by the X
chromosome its inheritance in man works out
on the same chromosome scheme and in the
same way as does white eye color (or any other
sex linked character) in the fly, for the O
sperm in man is equivalent to the Y sperm in
the fly.
In these cases we have been dealing with a
single pair of characters. Let us now take a
case where two pairs of sex linked characters
enter the cross at the same time, and preferably
a case where the two recessives enter the cross
from the same parent.
If a female with white eyes and yellow wings
is crossed to a wild male with red eyes and gray
wings (fig. 62), the sons are yellow and have
white eyes and the daughters are gray and
have red eyes. If two Fi flies are mated they
will produce the following classes.
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W.UJM wnrrr.^
GBl'v flrn ff
W W
@ff
4>
t
\ W /
ff
8"" (J
/.^i
i i'
YKLLOW WHITE GREY RED
■M
ftlLLOW RtD
Fig. 62. Cross between a white eyed, yellow winged female
of D. ampelophila and a red eyed, gray winged male. Two
pairs of sex linked characters, viz., white-red and yellow-gray
are involved. (See text.)
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THEORY OF EVOLUTION 129
Yellow Gray Yellow Gray
White Red Red White
99.% 1.%
Not only have the two grandparental combi-
nations reappeared, but in addition two new
combinations, viz., grey white and yellow red.
The two original combinations far exceed in
numbers the new or exchange combinations. If
we follow the history of the X chromosomes we
discover that the larger classes of grandchildren
appear in accdrd with the way in which the X
chromosomes are transmitted from one genera-
tion to the next.
The smaller classes of grandchildren, the ex-
change combinations or cross-overs, as we call
them, can be explained by the assumption
that at some stage in their history an inter-
change of parts has taken place between
the chromosomes. This is indicated in the
diagrams.
The most important fact brought out by the
experiment is that the factors that went in to-
gether tend to stick together. It makes no
difference in what combination the members of
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130 THEORY OF EVOLUTION
the two pairs of characters enter, they tend to
remain in that combination.
If one admits that the sex chromosomes carry
these factors for the sex-linked characters —
and the evidence is certainly very strong in
favor of this view — it follows necessarily from
these facts that at some time in their history
there has been an interchange between the two
sex chromosomes in the female.
There are several stages in the conjugation
of the chromosomes at which such an inter-
change between the members of a pair might
occur. There is further* a small amoimt of
direct evidence, unfortunately very meagre at
present, showing that an interchange does
actually occur.
At the ripening period of the germ cell the
members of each pair of chromosomes come to-
gether (fig. 49, e). In several forms they
have been described as meeting at one end and
then progressively coming to lie side by side as
shown in fig. 63, e, f, g, h, i. At the end of
the process they appear to have completely
united along their length (fig. 63, j, k, 1). It
is always a maternal and a paternal chromo-
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THEORY OF EVOLUTION
131
Fig. 63. Conjugation of chromosomes (side to side union) in
the spermatogenesis of Batracoseps. (After Janssens.)
some that meet in this way and always two of
the same kind. It has been observed that as
the members of a pair come together they oc-
casionally twist around each other (fig. 63, g,
1, and 64, and 65) . In consequence a part of one
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132 THEORY OF EVOLUTION
chromosome comes to be now on one side and
now on the other side of its mate.
When the chromosomes separate at the next
division of the germ cell the part on one side
passes to one pole, the part on the other to the
8888558888
oooc P H n m
Fig. 64. Scheme to illustrate a method of crossing over of
the chromosomes.
opposite pole, (figs. 64 and 65). Whenever
the chromosomes do not imtwist at this time
there must result an interchange of pieces
where they were crossed over each other.
Janssens has found at the time of separation
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THEORY OF EVOLUTION 133
evidence in favor of the view that some such
interchange probably takes place.
We find this same process of interchange of
characters taking place in each of the other
M
Fig. ^b. Scheme to illustrate double crossing over.
three groups of Drosophila. An example will
show this for the Group II.
If a black vestigial male is crossed to a gray
long- winged female (fig. 66) the offspring are
gray long. If an Fi female is back-crossed to
a black vestigial male the following kinds of
flies are produced :
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134 THEORY OF EVOLUTION
Black Gray Black Gray
vestigial long long vestigial
83% 17%
The combinations that entered are more com-
mon in the F2 generations than the cross-over
classes, showing that there is Unkage of the fac-
tors that entered together.
Another curious fact is brought out if in-
stead of back-crossing the Fi female we back-
cross the Fi male to a black vestigial female.
Their offspring are now of only two kinds,
black vestigial and gray long. This means
that in the male there is no crossing-over or
interchange of pieces. This relation holds not
only for the Group II but for all the other
groups as well.
Why interchange takes place in the female
of Drosophila and not in the male we do not
know at present. We might surmise that when
in the male the members of a pair come to-
gether they do not twist around each other,
hence no crossing-over results.
Crossing-over took place between white and
yellow only once in a hundred times. Other
characters show different values, but the same
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THEORY OF EVOLUTION 135
Mtes of Kale
OMCtM t r«Mj«
CjHD czid
CUD CUD CUD CUD
CUD dZD
CUD CUD
c
D
D
C
C
sy )
c
c
ty
iv
C
c
B..7.
iv
D
c
hv )
J
OUT TISTZOXiL
Fig. 66, Cross between black vestigial and gray long flies.
Two pairs of factors involved in the second group. The F^ fe-
male is back crossed (to right) to black vestigial male; and
the Fj male is back crossed to black vestigial female (to left).
Crossing over takes place in the Fj female but not in the
Fj male.
value under the same conditions is obtained
from the same pair of characters.
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Fig. 67.
ing those
studied.
Map of four chromosomes of D. ampelophila locat-
factors in each group that have been most fully
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THEORY OF EVOLUTION 187
If we assume that the nearer together the
factors lie in the chromosome the less likely is
a twist to occur between them, and conversely
the farther apart they lie the more likely is
a twist to occur between them, we can under-
stand how the linkage is different for different
pairs of factors.
On this basis we have made out chromosomal
maps for each chromosome (fig. 67) . The dia-
gram indicates those loci that have been most
accurately placed.
The Evidence from Interference
There is a considerable body of information
that we have obtained that corroborates the lo-
cation of the factors in the chromosome. This
evidence is too technical to take up in any de-
tail, but there is one result that is so important
that I must attempt to explain it. If, as I
assume, crossing over is brought about by twist-
ing of the chromosomes, and if owing to the
material of the chromosomes there is a most
frequent distance of intemode, then, when
crossing over between nodes takes place at
same level at a-b in figure 68, the region on
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188 THEORY OF EVOLUTION
each side of that point, a to A and b to B,
should be protected, so to speak, from further
crossing over. This in fact we have found to
be the case. No other explanation so far pro-
posed will account for this extraordinary
relation.
What advantage, may be asked, is there
in obtaining numerical data of this kind?
Fig. 68. Scheme to indicate that when the members of a
pair of chromosomes cross (at a-b) the region on each side is
protected inversely to the distance from a-b.
It is this: — ^whenever a new character appears
we need only determine in which of the four
groups it lies and its distance from two mem-
bers within that group. With this information
we can predict with a high degree of proba-
bility what results it will give with any other
member of any group. Thus we can do on
paper what would require many months of la-
bor by making the actual experiment. In a
word we can predict what will happen in a situ-
ation where prediction is impossible without
this numerical information.
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THEORY OF EVOLUTION 189
The Evidence from Non-Disjunction
In the course of the work on Drosophila ex-
ceptions appeared in one strain where certain
individuals did not conform to the scheme of
sex linked inheritance. For a moment the
hypothesis seemed to fail, but a careful exami-
nation led to the suspicion that in this strain
something had happened to the sex chromo^
somes. It was seen that if in some way the X
chromosomes failed to disjoin in certain eggs,
the exceptions could be explained. The analy-
sis led to the suggestion that if the Y chromo-
some had got into the female line the results
would be accounted for, since its presence there
would be expected to cause this peculiar non-
disjunction of the X chromosomes.
That this was the explanation was shown
when the material was examined. The females
that gave these results were found by Bridges
to have two X's and a Y chromosome.
The normal chromosome group of the fe-
male is shown in figure 52 and the chromosome
group of one of the exceptional females is
shown in figure 69. In a female of this kind
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140 THEORY OF EVOLUTION
there are three sex chromosomes X X Y
which are homologous in the sense that in nor-
mal individuals the two present are mates
and separate at the reduction division. If in
the X X Y individual X and X conjugate and
separate at reduction and the unmated Y is free
to move to either pole of the spindle, two kinds
of mature eggs will result, viz., X and XY.
If, on the other hand, X and Y conjugate and
9
Fig. 69. Figure of the chromosome group of an XXY
female, that gives non-disjunction.
separate at reduction and the remaining X is
free to go to either pole, four kinds of eggs will
result— XY—X— XX— Y. As a total result
four kinds of eggs are expected: viz. many
XY and X eggs and a few XX and Y eggs.
These four kinds of eggs may be fertilized
either by female-producing sperms or male-
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THEORY OF EVOLUTION
141
producing sperms, as indicated in the diagram
(fig. 70).
MY M XM XY
Fig. 70. Scheme showing the results of fertilizing white
bearing eggs (4 kinds) resulting from non-disjunction. The
upper half of the diagram gives the results when these eggs
are fertilized by normal red bearing, female producing sperm,
the lower half by normal, male producing s])erm.
If such an XXY female carried white bear-
ing Xs (open X in the figures), and the male
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142 THEORY OF EVOLUTION
carried a red bearing X (black X in the fig-
ures) it will be seen that there should result an
exceptional class of sons that are red, and
an exceptional class of daughters that are
white. Tests of these exceptions show that
they behave subsequently in heredity as their
composition requires. Other tests may also
be made of the other classes of offspring.
Bridges has shown that they fulfill all the re-
quirements predicted. Thus a result that
seemed in contradiction with the chromosome
hypothesis has turned out to give a brilliant
confirmation of that theory both genetically
and cytologically.
How Many Genetic Factors ake there in
THE Germ-plasm of a Single Individual
In passing I invite your attention to a
speculation based on our maps of the chromo-
somes — a speculation which I must insist does
not pretend to be more than a guess but has at
least the interest of being the first guess that
we have ever been in position to make as to
how many factors go towards the makeup of
the germ plasm.
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THEORY OF EVOLUTION 148
We have found practically no factors less
than .04 of a unit apart. If our map includes
the entire length of the chromosomes and if we
assume factors are uniformly distributed along
the chromosome at distances equal to the short-
est distance yet observed, viz. .04, then we
can calculate roughly how many hereditary
factors there are in Drosophila. The calcula-
tion gives about 7500 factors. The reader
should be cautioned against accepting the
above assumptions as strictly true, for crossing-
over values are known to differ according to
different environmental conditions (as shown
by Bridges for age), and to differ even in dif-
ferent parts of the chromosome as a result of
the presence of specific genetic factors (as
shown by Sturtevant). Since all the chromo-
somes except the X chromosomes are double
we must double our estimate to give the total
number of factors, but the half number is the
number of the different kinds of factors of
Drosophila.
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144 THEORY OF EVOLUTION
Conclusions
I have passed in review a long series of re-
searches as to the nature of the hereditary
material. We have in consequence of this
work arrived within sight of a result that
seemed a few years ago far beyond our reach.
The mechanism of heredity has, I think, been
discovered — discovered not by a flash of intui-
tion but as the result of patient and careful
study of the evidence itself.
With the discovery of this mechanism I
venture the opinion that the problem of hered-
ity has been solved. We know how the factors
carried by the parents are sorted out to the
germ cells. The explanation does not pretend
to state how factors arise or how they influ-
ence the development of the embryo. But
these have never been an integral part of the
doctrine of heredity. The problems which they
present must be worked out in their own field.
So, I repeat, the mechanism of the chromo-
somes offers a satisfactory solution of the tra-
ditional problem of heredity.
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CHAPTER IV
SELECTION AND EVOLUTION
Darwin's Theory of Natural Selection still
holds today first place in every discussion of
evolution, and for this very reason the theory
calls for careful scrutiny; for it is not difficult
to show that the expression "natural selection"
is to many men a metaphor that carries many
meanings, and sometimes different meanings to
different men. While I heartily agree with my
fellow biologists in ascribing to Darwin him-
self, and to his work, the first place in biological
philosophy, yet recognition of this claim should
not deter us from a careful analysis of the situ-
ation in the light of work that has been done
since Darwin's time.
The Theory of Natural Selection
In his great book on the Origin of Species,
Darwin tried to do two things: first, to show
that the evidence bearing on evolution makes
145
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146 THEORY OF EVOLUTION
that explanation probable. No such great
body of evidence had ever been brought to-
gether before, and it wrought, as we all know,
a revolution in our modes of thinking.
Darwin also set himself the task of showing
how evolution might have taken place. He
pointed to the influence of the environment, to
the effects of use and disuse, and to natural
selection. It is to the last theory that his name
is especially attached. He appealed to a fact
familiar to everyone, that no two individuals
are identical and that some of the differences
that they show are inherited. He argued that
those individuals that are best suited to their
environment are the most probable ones to sur-
vive and to leave most offspring. In conse-
quence their descendants should in time replace
through competition the less well-adapted
individuals of the species. This is the process
Darwin called natural selection, and Spencer
the survival of the fittest.
Stated in these general terms there is noth-
ing in the theory to which anyone is likely to
take exception. But let us examine the argu-
ment more critically.
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THEORY OF EVOLUTION
147
If we measure, or weigh, or classify any-
character shown by the individuals of a popu-
lation, we find differences. We recognize that
some of the differences are due to the varied
experiences that the individuals have encoun-
Fio. 71. Series of leaves of a tree arranged according to
size. (After de Vries.)
tered in the course of their lives, i.e. to their en-
vironment, but we also recognize that some of
the differences may be due to individuals hav-
ing different inheritances — different germ
plasms. Some familiar examples will help
to bring home this relation.
If the leaves of a tree are arranged accord-
ing to size (fig. 71), we find a continuous ser-
ies, but there are more leaves of medium size
than extremes. If a lot of beans be sorted out
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148
THEORY OF EVOLUTION
according to their weights, and those between
certain weights put into cylinders, the cylin-
ders, when arranged according to the size of the
beans, will appear as shown in figure 72. An
imaginary line running over the tops of the
Fig. 72. Beans put into cylindrical jars according to the
sizes of the beans. The jars arranged according to size of
contained beans. (After de Vries.)
piles will give a curve (fig. 73) that corresponds
to the curve of probability (fig. 74) .
If we stand men in lines according to their
height (fig. 75) we get a similar arrangement.
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THEORY OF EVOLUTION
149
41 r t
Fig. 73. A curve resulting from arrangement of beans
according to size. (After de Vries.)
The differences in size shown by the individ-
ual beans or by the individual men are due in
part to heredity, in part to the environment
-3d *2<5 *d +(5 *26 +3(5
Fig. 74. Curve of probability. (After Johannsen.)
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150
THEORY OF EVOLUTION
*^i
in which they have developed. This is a famil-
iar fact of almost every-day observation. It
is well shown in the following example. In
figure 76 the two boys and the two varieties of
Fig. 75. Students arranged according to size. (After
Blakeslee.)
corn, which they are holding, differ in height.
The pedigrees of the boys (fig. 77) make it
probable that their height is largely inherited
and the two races of corn are known to belong
to a tall and a short race respectively. Here,
then, the chief effect or difference is due to
heredity. On the other hand, if individuals of
the same race develop in a favorable environ-
ment the result is different from the develop-
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THEORY OF EVOLUTION
151
ment in an unfavorable environment, as shown
in figure 78. Here to the right the corn is
en
S ^
a >
c ^ x»
BD O °
^ m r-
> as 5
» -» 5.
•< > <
•r ^
Fig. 76. A short and a tall boy each holding a stalk of corn —
one stalk of a race or short corn, the other of tall corn. (After
Blakeslee. )
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152 THEORY OF EVOLUTION
crowded and in consequence dwarfed, while to
the left the same kind of com has had more
room to develop and is taller.
Darwin knew that if selection of particular
kinds of individuals of a population takes place
the next generation is affected. If the taller
men of a community are selected the average of
their offspring will be taller than the average of
W 5'5"k
Fig. 77. Pedigree of boys shown in Fig. 76. (After
Blakeslee.)
the former population. If selection for tallness
again takes place, still taller men will on the
average arise. If, amongst these, selection
again makes a choice the process would, he
thought, continue (fig. 79).
We now recognize that this statement con-
tains an important truth, but we have found that
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THEORY OF EVOLUTION
153
i
Fig. 78. Corn reared under different conditions. (After
Blakeslee.)
it contains only a part of the truth. Any one
who repeats for himself this kind of selection
experiment will find that while his average class
will often change in the direction of his selec-
tion, the process slows down as a rule rather
suddenly (fig. 80). He finds, moreover, that
the limits of variability are not necesarily trans-
cended as the process continues even although
the average may for a while be increased. More
tall men may be produced by selection of this
kind, but the tallest men are not necessarily any
taller than the tallest in the original population.
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154
THEORY OF EVOLUTION
Selection, then, has not produced anything
new, but only more of certain kinds of indi-
FiG. 79. Curves showing how (hypothetically) selection might
be supposed to bring about progress in direction of selection.
(After Goldschmidt.)
viduals. Evolution, however, means producing
more new things, not more of what already
exists.
Darwin seems to have thought that the range
of variation shown by the offspring of a given
individual about that type of individual would
be as wide as the range shown by the original
population (fig. 79), but Galton's work has
made it clear that this is not the case in a gen-
eral or mixed population. If the offspring of
individuals continued to show, as Darwin seems
to have thought, as wide a range on each side of
their parents' size, so to speak, as did the orig-
inal population, then it would follow that se-
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THEORY OF EVOLUTION
155
lection could slide successive generations along
in the direction of selection.
Darwin himself was extraordinarily careful,
however, in the statements he made in this con-
nection and it is rather by implication than by
actual reference that one can ascribe this mean-
FiG. 80. Diagram illustrating the results of selection for
extra bristles in D. ampelophila. Selection at first produces
decided eflFects which soon slow down and then cease.
(MacDowell.)
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156 THEORY OF EVOLUTION
ing to his views. His contemporaries and
many of his followers, however, appear to have
accepted this sliding scale interpretation as
the cardinal doctrine of evolution. If this is
doubted or my statement is challenged then
one must explain why de Vries' mutation the-
ory met with so little enthusiasm amongst the
older group of zoologists and botanists; and
one must explain why Johannsen's splendid
work met with such bitter opposition from
the English school — the biometricians — who
amongst the post-Darwinian school are as-
sumed to be the lineal descendants of Darwin.
And in this connection we should not forget
that just this sort of process was supposed to
take place in the inheritance of use and disuse.
What is gained in one generation forms the
basis for further gains in the next generation.
Now, Darwin not only believed that acquired
characters are inherited but turned more and
more to this explanation in his later writings.
Let us, however, not make too much of the
matter ; for it is much less important to find out
whether Darwin's ideas were vague, than it is
to make sure that our own ideas are clear.
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THEORY OF EVOLUTION 157
If I have made several statements here that
appear dogmatic let me now attempt to justify
them, or at least give the evidence which seems
to me to make them probable.
The work of the Danish botanist, Johannsen,
has given us the most carefully analyzed case
of selection that has ever been obtained. There
are, moreover, special reasons why the mater-
ial that he used is better suited to give definite
information than any other so far studied.
Johannsen worked with the common bean,
weighing the seeds or else measuring them.
These beans if taken from many plants at ran-
dom give the typical curve of probability (fig.
74) . The plant multiplies by self-fertilization.
Taking advantage of this fact Johannsen kept
the seeds of each plant separate from the
others, and raised from them a new generation.
When curves were made from these new groups
it was found that some of them had different
modes from that of the original general popula-
tion (fig. 81 A-E, bottom group). They
are shown in the upper groups (A, B, C, D,
E) . But do not understand me to say that the
offspring of each bean gave a different mode.
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158
Fig. 81. Pure lines of beans. The lower figure gives the
general population, the other figures give the pure lines within
the population. (After Johannsen.)
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THEORY OF EVOLUTION 159
On the contrary, some of the lines would be
the same.
The result means that the general population
is made up of definite kinds of individuals that
may have been sorted out.
That his conclusion is correct is shown by
rearing a new generation from any plant or in-
deed from several plants of any one of these
lines. Each line repeats the same modal class.
There is no further breaking up into groups.
Within the line it does not matter at all whether
one chooses a big bean or a little one — they will
give the same result. In a word, the germ
plasm in each of these lines is pure, or homo-
zygous, as we say. The differences that we find
between the weights (or sizes) of the individual
beans are due to external conditions to which
they have been subjected.
In a word, Johannsen's work shows that the
frequency distribution of a pure line is due to
factors that are extrinsic to the germ plasm.
It does not matter then which individuals in a
pure line are used to breed from, for they all
carry the same germ plasm.
We can now understand more clearly how
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160 THEORY OF EVOLUTION
selection acting on a general population brings
about results in the direction of selection.
An individual is picked out from the popu-
lation in order to get a particular kind of germ
plasm. Although the different classes of indi-
viduals may overlap, so that one can not alTvays
judge an individual from its appearance, never-
theless on the whole chance favors the picking
out of the kind of germ plasm sought.
In species with separate sexes there is the
further difficulty that two individuals must be
chosen for each mating, and superficial examin-
ation of them does not insure that they belong
to the same group — ^their germ plasm cannot
be inspected. Hence selection of biparental
forms is a precarious process, now going for-
ward, now backwards, now standing still. In
time, however, the process forward is almost
certain to take place if the selection is from
a heterogeneous population. Johannsen's
work was simplified because he started with
pure lines. In fact, had he not done so his
work would not have been essentially different
from that of any selection experiment of a pure
race of animals or plants. Whether Johannsen
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THEORY OF EVOLUTION 161
realized the importance of the condition or
not is uncertain — curiously he laid no em-
phasis on it in the first edition of his "Elemente
der exakten Erblichkeitslehre".
It has since been pointed out by Jennings
and by Pearl that a race that reproduces by
self-fertilization as does this bean, automati-
cally becomes pure in all of the factors that
make up its germ plasm. Since self-fertiliza-
tion is the normal process in this bean the pur-
ity of the germ plasm already existed when
Johannsen began to experiment.
How Has Selection in Domesticated
Animals and Plants Brought About
Its Results?
If then selection does not bring about trans-
gressive variation in a general population, how
can selection produce anything new? If it
can not produce anything new, is there any
other way in which selection becomes an agent
in evolution?
We can get some light on this question if we
turn to what man has done with his domesti-
cated animals and plants. Through selection.
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162 THEORY OF EVOLUTION
i.e., artificial selection, man has undoubtedly
brought about changes as remarkable as any
shown by wild animals and plants. We know,
moreover, a good deal about how these changes
have been wrought.
( 1 ) By crossing different wild species or by
crossing wild with races already domesticated
new combinations have been made. Parts of
one individual have been combined with parts
of others, creating new combinations. It is
possible even that characters that are entirely
new may be produced by the interaction of fac-
tors brought into recombination.
(2) New characters appear from time to
time in domesticated and in wild species.
These, like the mutants in Drosophila, are fully
equipped at the start. Since they breed true
and follow Mendel's laws it is possible to com-
bine them with characters of the wild type or
with those of other mutant races.
Amongst the new mutant factors there
may be some whose chief effect is on the char-
acter that the breeder is already selecting.
Such a modification will be likely to attract at-
tention. Superficially it may appear that the
Digitized by
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THEORY OF EVOLUTION 163
factor for the original character has varied,
while the truth may be that another factor has
appeared that has modified a character already
present. In fact, many or all Mendelian fac-
tors that affect the same organ may be said
to be modifiers of each other's effects. Thus
the factor for vermilion causes the eye to be
one color, and the factor for eosin another
color, while eosin vermilion is different from
both. Eosin may be said to be a modifier of
vermilion or vermilion of eosin. In general,
however, it is convenient to use the term "modi-
fier" for cases in which the factor causes a
detectable change in a character already pres-
ent or conspicuous.
One of the most interesting, and at the same
time most treacherous, kinds of modifying
factors is that which produces an effect only
when some other factor is present. Thus
Bridges has shown that there is a factor called
"cream" that does not affect the red color of
the eye of the wild fly, yet makes "eosin" much
paler (fig. 82). Another factor "whiting"
which produces no effect on red makes eosin en-
tirely white. Since cream or whiting may be
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164 THEORY OF EVOLUTION
carried by red eyed flies without their presence
being seen until eosin is used, the experimenter
must be continually on the lookout for such
factors which may lead to erroneous conclusions
Red
- o _
cclcreamJ ee ww[whitindl
o o
eecc eeww
Fig. 89. Scheme to indicate influence of the modifying fac-
tors, cream and whiting. Neither produces any effect alone but
they modify other eye colors such as eosin.
unless detected. As yet breeders have not
realized the important role that modifiers
have played in their results, but there are indi-
cations at least that the heaping up of modify-
ing factors has been one of the ways in which
Digitized by
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THEORY OF EVOLUTION 165
highly specialized domesticated animals have
been produced. Selection has accomplished
this result not by changing factors, but by
picking up modifying factors. The demonstra-
tion of the presence of these factors has already
been made in some cases. Their study prom-
ises to be one of the most instructive fields for
further work bearing on the selection hypo-
thesis.
In addition to these well recognized methods
by which artificial selection has produced new
things we come now to a question that is the
very crux of the selection theory today. Our
whole conception of selection turns on the an-
swer that we give to this matter and if I appear
insistent and go into some detail it is because I
think that the matter is worth very careful
consideration.
Are Factors Changed Through
Selection?
As we have seen, the variation that we find
from individual to individual is due in part
to the environment; this can generally be
demonstrated. Other differences in an or-
Digitized by
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166 THEORY OF EVOLUTION
dinary population are recognized as due to dif-
ferent genetic (hereditary) combinations.
No one will dispute this statement. But is all
the variabihty accounted for in these two ways ?
May not a factor itself fluctuate? Is it not
a priori probable that factors do fluctuate?
Why, in a word, should we regard factors as in-
violate when we see that everything else in
organisms is more or less in amount? I do not
know of any a priori reason why a factor may
not fluctuate, unless it is, as I hke to think, a
chemical molecule. We are, however, dealing
here not with generahties but with evidence,
and there are three known methods by means
of which it has been shown that variabihty,
other than environmental or recombinational,
is not due to variabihty in a factor, nor to vari-
ous "potencies" possessed by the same factors.
(1) By making the stock uniform for all of
its factors — chief factors and modifiers alike.
Any change in such a stock produced by selec-
tion would then be due to a change in one or
more of the factors themselves. Johannsen's
experiment is an example of this sort.
(2) The second method is one that is capa-
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THEORY OF EVOLUTION 167
ble of demonstrating that the effects of selec-
tion are actually due to modifiers. It has been
Fig. 83 a. Drosophila ampelophila with truncate wings.
worked out in our laboratory, chiefly by Mul-
ler, and used in a particular case to demonstrate
that selection produced its effect by isolating
modifying factors. For example, a mutant
type called truncate appeared, characterized
by shorter wings, usually square at the end,
(fig. 83a). The wings varied from those of
normal length to wings much shorter (fig.
83b). For three years the mutant stock was
Digitized by
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168 THEORY OF EVOLUTION
Fig. 83 b. Series of wings of different length shown by trun-
cate stock of D. ampelophila.
bred from individuals having the shorter
wings until at last a stock was obtained in
which some of the individuals had wings much
shorter than the body. By means of linkage
experiments it was shown that at least three
factors were present that modified the wings.
These were isolated by means of their linkage
relations, and their mutual influence on the pro-
duction of truncate wings was shown.
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THEORY OF EVOLUTION 169
An experiment of this kind can only be car-
ried out in a case where the groups of linked
gens are known. At present Drosophila is
the only animal (or plant) sufficiently well
known to make this test possible, but thijs does
not prove that the method is of no value. On
the contrary it shows that any claim that factors
can themselves be changed can have no finality
until the claim can be tested out by means of
the linkage test. For instance, bar eye (fig.
31 ) arose as a mutation. All our stock has de-
scended from a single original mutant. But
Zeleny has shown that selection within oui*
stock will make the bar eye narrower or
broader according to the direction of selection.
It remains to be shown in this case how selec-
tion has produced its effects, and this can be
done by utilizing the same process that was
used in the case of truncate.
Another mutant stock called beaded (fig.
84), has been bred for five years and selected
for wings showing more beading. In extreme
cases the wings have been reduced to mere
stumps (see stumpy, fig. 5), but the stock
shows great variability. It is probable here
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170
THEORY OF EVOLUTION
as Dexter has shown, that a number of mutant
factors that act as modifiers have been picked
Fig. 84. Two flies showing beaded wings.
up in the course of the selection, and when it
is recalled that during those five years over
125 new characters have appeared elsewhere
it does not seem improbable that factors also
have appeared that modify the wings of this
stock.
(3) The third method is one that has been
developed principally by East for plants; also
by MacDowell for rabbits and flies. The
Digitized by VjOOQIC
THEORY OF EVOLUTION
171
method does not claim to prove that modifiers
are present, but it shows why certain results
are in harmony with that expectation and can
not be accounted for on the basis that a factor
has changed. Let me give an example. When
a Belgian hare with large body was crossed to
a common rabbit with a small body the hybrid
was intermediate in size. When the hybrid
was crossed back to the smaller type it pro-
duced rabbits of various sizes in apparently
a continuous series. MacDowell made meas-
urements of the range of variability in the first
and in the second generations.
He found that the variability was smaller in
the first generation than in the second genera-
Classiflcation in relation to parents based on skull lengths and ulna
lengths^ to show the relative variability of two measurements and
of the first generation (Fi) and the back cross {B. C.)
(TRABAOTBR
R A'FTON
Lpngth of i Fi
skull { ; BM.
Loii Bth of j
ulna j
-I2-
■uL
10
-b|-8^1
'7-eh
ikoUU!'!
S B i
-m.
1
* 4V
8 4U
iimim
aosp
\fn
Tjimi^th of S
rfJEUlI 1 I
Lffiiirth of J
uhia 1 ,
Fi
B.O.
same taH>' cmttinued
I I
BATIOX
11 1
IS 1.1
It^ie
sIlOl 7 S 3] 2^
ti, k a' 1' !
2 5; 3 1, 7 3| 31 1
Digitized by
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172 THEORY OF EVOLUTION
tion (back cross). This is what is expected if
several factor-differences were involved, be-
cause the hybrids of the first generation are
expected to be more uniform in factorial com-
position than are those in the second generation
which are produced by recombination of the
factors introduced through their grandparents.
Excellent illustrations of the same kinds of
results have been found in Indian com. As
shown in figure 85 the length of the cob in Fi
is intermediate between the parent types while
in F2 the range is wider and both of the original
types are recovered. East states that similar
relations have been found for 18 characters
in corn. Emerson has recently furnished
further illustrations of the same relations in
the length of stalks in beans.
A similar case is shown by a cross between
fantail and common pigeons (fig. 86). The
latter have twelve feathers in the tail, while the
selected race from which the f antails came had
between 28 and 38 feathers in the tail. The Fi
offspring (forty-one individuals) showed (fig.
87) between 12 and 20 tail feathers, while
in F2 the numbers varied between 12 and
Digitized by
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THEORY OF EVOLUTION 173
i-miui
ull
iilUIL
4iiiiiiuiiii
Fig. 85. Cross between two races of Indian corn, one with
short cobs and one with long cobs. The range of variability in
F^ is less than that in F^. (After East.)
Digitized by
Googk
Fig. 86. Cross of pigeon with normal tail P, and fantail P,;
F, bird below.
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THEORY OF EVOLUTION
175
25. Here one of the grand-parental types
reappears in large numbers, while the extreme
of the other grand-parental type did not re-
appear (in the counts obtained), although the
!F2 number would probably overlap the lower
-L
J_L
Fi
. 1
. 1
i .
u ■ » ■
Bart cross
■ Il.l
Fig. 87. Cross of normal and fantail pigeons. (See Fig.
86.) The Fg range is wider than that of Fj. The normal grand-
parental type of 12 feathers was recovered in F2 but the higher
numbers characteristic of fantails were not recovered.
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176
THEORY OF EVOLUTION
limits of the race of f antail grandparents had
not a selected (surviving) lot been taken for
the figures given in the table.
The preceding account attempts to point out
how I should prefer to interpret the problem
of selection in the light of the most recent work
on breeding. But I would give a very incom-
plete account of the whole situation if I neg-
lected to include some important work which
has led some of my fellow-workers to a very
different conclusion.
Castle in particular is the champion of a
view based on his results with hooded rats.
Fig. 88. Scheme to show classes of hooded rats used by
Castle. (After Castle.)
Starting with individuals which have a narrow
black stripe down the back he selected for a
narrower stripe in one direction and for a
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THEORY OF EVOLUTION 177
broader stripe in the other. As the diagram
shows (fig. 88) Castle has succeeded in pro-
ducing in one direction a race in which the
dorsal stripe has disappeared and in the other
direction a race in which the black has extended
over the back and sides, leaving only a white
mark on the belly. Neither of these extremes
occurs, he believes, in the ordinary hooded race
of domesticated rats. In other words no mat-
ter how many of them came under observa-
tion the extreme types of his experiment would
not be found.
Castle claims that the factor for hoodedness
must be a single Mendelian unit, because if
hooded rats are crossed to wild gray rats with
imiform coat and their offspring are inbred
there are produced in F2 three imiform rats to
one hooded rat. Castle advances the hypothe-
sis that factors — by which he means Mendelian
factors — may themselves vary in much the
same way as do the characters that they stand
for. He argues, in so many words, that since
we judge a factor by the kind of character it
produces, when the character varies the factor
that stands for it may have changed.
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178 THEORY OF EVOLUTION
As early as 1903 Cuenot had carried out
experiments with spotted mice similar to those
of Castle with rats. Cuenot found that spotted
crossed to uniform coat color gave in F2 a ratio
of three uniform to one spotted, yet selection
of those spotted mice with more white in their
coat produced mice in successive generations
that had more and more white. Conversely
Cuenot showed that selection of those spotted
mice that had more color in their coat produced
mice with more and more color and less white.
Cuenot does not however bring up in this con-
nection the question as to how selection in these
spotted mice brings about its results.
Without attempting to discuss these results
at the length that they deserve let me briefly
state why I think Castle's evidence fails to es-
tablish his conclusion.
In the first place one of the premises may be
wrong. The three to one ratio in F2 by no
means proves that all conditions of hoodedness
are due to one factor. The result shows at most
that one factor that gives the hooded types is
a simple Mendelian factor. The changes in
this type may be caused by modifying factors
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THEORY OF EVOLUTION 179
that can show an effect only when hoodedness
is itself present. That this is not an imaginary
objection but a real one is shown by an experi-
ment that Castle himself made which furnishes
the ground for the second objection.
Second. If the factor has really changed its
potency, then if a very dark individual from
one end of the series is crossed to a wild rat and
the second generation raised we should expect
that the hooded F2 rats would all be dark like
their dark grandparent. When Castle made
this test he found that there were many grades
of hooded rats in the Fg progeny. They were
darker, it is true, as a group than were the
original hooded group at the beginning of the
selection experiment, but they gave many in-
termediate grades. Castle attempts to explain
this by the assumption that the factor made
pure by selection became contaminated by its
normal allelomorph in the Fi parent, but not
only does this assumption appear to beg the
whole question, but it is in flat contradiction
with what we have observed in hundreds of
Mendelian cases where no evidence for such
a contamination exists.
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180 THEORY OF EVOLUTION
Later Castle crossed some of the extracted
rats of average grade (3.01) from the plus
series to the same wild race and got F2 hooded
rats from this cross. These F2 hooded rats did
not further approach the ordinary range but
were nearer the extreme selected plus hooded
rats (3.33) than were the F2's extracted from
the first cross (2.59). Castle concludes from
this that multiple factors can not account for
the result. As a matter of fact, Castle's evi-
dence as published does not establish his con-
clusion because the wild rats used in the second
experiment may have carried plus modifiers.
This could only be determined by suitable tests
which Castle does not furnish. This is the
crucial point, without which the evidence carries
no conviction.
Furthermore, from Castle's point of view,
these latest results would seem to increase the
difficulty of interpretation of his first F2 ex-
tracted cross, and it is now the first result that
calls for explanation if one accepts his later
conclusion.
These and other objections that might be
taken up show, I think, that Castle's experi-
Digitized by
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THEORY OF EVOLUTION
181
merit with hooded rats fails entirely to estab-
lish his contention of change in potency of the
germ or of contamination of factors, while on
the contrary they are in entire accord with the
view that he is dealing with a case of modify-
ing factors.
Equally important are the results that Jen-
nings has obtained with certain protozoa. Par-
amecium multiplies by dividing across in the
5909««««
99a6M
Fig. 89. Races of Paramecium. (After Jennings.)
Digitized by
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182 THEORY OF EVOLUTION
middle, each half replacing its lacking part.
Both the small nucleus (micronucleus) and the
large nucleus (macronucleus) divide at each
division of the body. Jennings found that
while individuals descended from a single
Paramecium vary in size (fig. 89), yet the
population from a large individual is the same
as the population derived from a small individ-
ual. In other words, selection produces no re-
sult and the probable explanation is, of course,
that the different sizes of individuals are due
to the environment, while the constancy of the
type is genetic. Jennings foimd a number of
races of paramecium of different sizes living
under natural conditions. The largest indi-
vidual of a small race might overlap the small-
est individual of other larger races (fig. 89) ;
nevertheless each kind reproduced its particu-
lar race. The results are like those of Johann-
sen in a general way, but differ in that repro-
duction takes place in paramecium by direct
division instead of through self-fertilization as
in beans, and also in that the paramecia were
probably not homozygous. Since, however,
so far as known no "reduction" tr.kes place in
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THEORY OF EVOLUTION
183
Paramecium at each division, the genetic com-
position of parent and offspring should be the
same. Whether pseudo-parthenogenesis that
Woodruff and Erdmann have foimd occurring
in Paramecium at intervals involves a redistri-
bution of the hereditary factors is not clear.
Jennings's evidence seems incompatible with
such a view.
More recently one of Jennings's students,
Middleton, has made a careful series of selec-
tion experiments with Stylonychia (fig. 90) in
which he selected for lines showing more rapid
•ticu-
ireei
ver.
eiii
Fig. 90. Stylonychia showing division into two. (After
Stein.)
Digitized by
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184 THEORY OF EVOLUTION
or slower rates of division. His observations
seem to show that his selection separated two
such lines that came from the same original
stock. The rapidity of the effects of selection
seems to preclude the explanation that pseudo-
parthenogenesis has complicated the results.
Nevertheless, the results are of such a kind as
to suggest that they were due to selection of
vegetative (somatic) differences and that no
genetic change of factors was involved, for his
conclusion that the rapidity with which the
effects gained by long selection might be sud-
denly reversed when selection was reversed is
hardly consistent with an interpretation of the
results based on changes in the "potencies" of
the factors present.
Equally striking are the interesting experi-
ments that Jennings has recently carried out
with Difflugia (fig. 91). This protozoon se-
cretes a shell about itself which has a charac-
teristic shape, and often carries spines. The
opening at one end of the shell through which
the protoplasm protrudes to make the pseudo-
podia is surrounded by a rim having a charac-
teristic pattern. The protoplasm contains
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THEORY OF EVOLUTION 185
several nuclei and in addition there is scattered
material or particles called chromidia that are
Fig. 91. DifHugia corona. (After Cash.)
supposed to be chromatic in nature and related
to the material of the nuclei, possibly by direct
interchange.
When Difflugia divides, part of the proto-
plasm protrudes from the opening and a new
shell is secreted about this mass which becomes
a daughter individual. The behavior of the
nucleus and of the chromidia at this time is ob-
scure, but there is some evidence that their
materials may be irregularly distributed be-
Digitized by
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186 THEORY OF EVOLUTION
tween parent and offspring. If this is correct,
and if in the protozoa the chromatin has the
same influence that it seems to have in higher
animals, the mode of reproduction in DiflBugia
would be expected to give little more than ran-
dom sampling of the germ plasm.
Jennings was able by means of selection to
get from the descendants of one original indi-
FiG. 92. Races of Difflugia. (After Leidy.)
vidual a number of different types that them-
selves bred true, except in so far as selection
could affect another change in them. In this
connection it is interesting to note that Leidy
Digitized by
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THEORY OF EVOLUTION 187
has published figures of DiflBugia (fig. 92) that
show that a great many "types" exist. If
through sexual union (a process that occurs in
DiflBugia) the germ plasm (chromatin) of these
wild types has in times past been recombined,
then selection would be expected to separate
certain types again, if, at division, irregular
sampling of the germ plasm takes place. Until
these points are settled the bearing of these
important experiments of Jennings on the
general problem of selection is uncertain.
How Does Natural Selection Influence
THE Course or Evolution?
The question still remains: Does selection
play any role in evolution, and, if so, in what
sense? Does the elimination of the unfit influ-
ence the course of evolution, except in the nega-
tive sense of leaving more room for the fit?
There is something further to be said in this
connection, although opinions may diflFer as to
whether the following interpretation of the
term "natural selection" is the only possible
one.
If through a mutation a character appears
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188
THEORY OF EVOLUTION
that is neither advantageous nor disadvanta-
geous, but indifferent, the chance that it may
become established in the race is extremely
small, although by good luck such a thing may
occur rarely. It makes no difference whether
the character in question is a dominant or a
Phutoccnr
l^ftfierFhocene
tUPHAS
(almtcktn)
r Pliocene TETRABELOOt't*
MiddkMiocme TETRABtLOfXm
(ANCJU»TIOrNS SIACC}
l/mtr Miocene (fm^ehinj
" ' oUo Europe - Aua
* PALACOMASTOOON
(Imiflkmmf ehim)
HOCKtTNCKlUM
t^ifterEeeene
l^f
Fig. 93. Evolution of elephant's skulls. (After Dendy.)
Digitized by
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THEORY OF EVOLUTION 189
recessive one, the chance of its becoming estab-
lished is exactly the same. If through a muta-
tion a character appears that has an injurious
effect, however slight this may be, it has prac-
tically no chance of becoming established.
If through a mutation a character appears
Fig. 94. Evolution of elephant's trunk. (After Lull.)
that has a beneficial influence on the individual,
the chance that the individual will survive is
increased, not only for itself, but for all of its
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190 THEORY OF EVOLUTION
descendants that come to inherit this character.
It is this increase in the number of individuals
possessing a particular character, that might
have an influence on the course of evolution.
This gives a better chance for improvement by
several successive steps; but not because the
species is more likely to mutate again in the
same direction. An imaginary example will
illustrate how this happens: When elephants
had trunks less than a foot long, the chance of
getting trunks more than one foot long was
in proportion to the length of trunks already
present and to the number of individuals; but
increment in trunk length is no more likely to
occur from an animal having a trunk more than
one foot long than from an animal with a
shorter trunk.
The case is analogous to tossing pennies. At
any stage in the game the chance of accumu-
lating a hundred heads is in proportion to the
number of heads already obtained, and to the
number of throws still to be made. But the
number of heads obtained has no influence on
the number of heads that will appear in the next
throw.
Digitized by
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Fig. 95. Evolution of elephant's trunk: above Maeritherium,
ii the middle Tetrabelodon (After Lancaster); below African
elephant (After Gambler Bolton).
Digitized by
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192 THEORY OF EVOLUTION
Owing then to this property of the germ
plasm to duplicate itself in a large nmnber of
samples not only is an opportmiity furnished to
an advantageous variation to become exten-
sively multiplied, but the presence of a large
number of individuals of a given sort preju-
dices the probable future result.
The question may be raised as to whether it
is desirable to call selection a creative process.
There are so many supernatural and mystical
implications that hang around the term creative
that one can not be too careful in stating in
what sense the term is to be used. If by crea-
tive is meant that something is made out of
nothing, then of course there is no need for the
scientist to try to answer such a question. But
if by a creative process is meant that something
is made out of something else, then there are
two alternatives to be reckoned with.
First, if it were true that selection of an indi-
vidual of a certain kind determines that new
variations in the same direction occur as a con-
sequence of the selection, then selection would
certainly be creative. How this could occur
might be quite unintelligible, but of course it
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THEORY OF EVOLUTION 193
might be claimed that the point is not whether
we can explain how creation takes place, but
whether we can get verifiable evidence that such
a kind of thing happens. This possibility is
disposed of by the fact that there is no evidence
that selection determines the direction in whicli
variation occurs.
Second, if you mean by a creative process
that by picking out a certain kind of individual
and multiplying its numbers a better chance is
furnished that a certain end result will be ob-
tained, such a process may be said to be crea-
tive. This is, I think, the proper use of the
term creative in a mechanistic sense.
Conclusions
In reviewing the evidence relating to selec-
tion I have tried to handle the problem as ob-
jectively as I could.
The evidence shows clearly that the charac-
ters of wild animals and plants, as well as those
of domesticated races, are inherited both in the
wild and in the domesticated forms according
to Mendel's Law.
The causes of the mutations that give rise
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194 THEORY OF EVOLUTION
to new characters we do not know, although
we have no reason for supposing that they are
due to other than natural processes.
Evolution has taken place by the incorpora-
tion into the race of those mutations that are
beneficial to the life and reproduction of the
organism. Natural selection as here defined
means both the increase in the number of
individuals that results after a beneficial muta-
tion has occurred (owing to the ability of living
matter to propagate) and also that this prepon-
derance of certain kinds of individuals in a
population makes some further results more
probable than others. More than this, natural
selection can not mean, if factors are fixed and
are not changed by selection.
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INDEX
Abnormal abdomen 109
Abraxas 78-81
Allan tois 17
Allelomorphs 83-84
Altenburg 112
Amnion 16-17
Andalusian fowl 45, 46
Annelids 29
Antlered wing 111
Apterous wing 11
Arc wing 111
Aristae 104
Bar eye 67, 108, 169
Bateson 18, 34, 36
Beaded wing 11, 115
Beans 147-149, 157
Belgian hare 171
Bent wing 116
Bergson 30, 31
Bildungstrieb 34
Biogenetic law 15, 18, 19, 21
Biometricians 156
Bird 21, 23
Bithorax 65, 112, 113,
Black body color 111, 133
Blakeslee 152
Bridges 114, 143, 163
British Association 36
Briinn 40
Buif eye color 109
Buifon 27
Castle. 176-180
Cat 33
Cell 90, 91
Chance variations 37
Chick 16, 17, 20
Chromatin 184
Chromosome group of Drosoph-
ila 102
Chromosomes 91, 95, 96, 98, 130,
131, 132
Cleavage 21, 22, 94
Clover butterfly 62
Club wing 69, 70, 108
Colias philodice 62
Color blindness 77, 125
Comb of Drosophila 103
Combs of fowls 33, 54
Comparative anatomy 7, 8, 9,
14
Corn 150, 153, 172
Correns 41
Cosmogonies 27
Cream eye color 163, 164
Crepidula 22
Criss-cross inheritance 78
Crossing over 131-133
Cu6not 178
Curled wing 115
Curved wing 111
Curve of probability 149
Cut wing 11, 104
Dachs legs 112
Dahlgren 90, 92
Darwin 15, 24, 28, 32, 36-39, 64,
145, 146, 152, 154-156
Dendy 188
De Vries 18, 147, 156
Dexter 170
Dichaete 114
Difflugia 184-187
Discontinuous variation 13
Disuse 31
Drosophila ampelophila 10, 12,
195
Digitized by
Googk
196
INDEX
13, 48-52, 60, 75, 84, 85, 93,Hoge 66
100, 103, 119, 155, 162, 169 Horse, evolution of, 6
Drosophila repleta 75
Duplication of legs 109 Indian com 172, 173
Dwarf 114 Interference 137, 138
East 170, 172
Ebony 50, 55, 56, 115
Egg 91, 94
Elephant 191
Elephants' skulls 188
Elephants' trunks 190
Embryology 14-23
Emerson 172
Environment 27
Eosin eye color 61, 107, 163
Erdmann 183
Evolution Creatrice 30
Evolution — three kinds of —
1, 2, 4
Eye color 13
Eyeless 66, 116
Factorial theory 89
Factors of Drosophila 143
Fantails 172, 175
Fertilization 91
Fish 16, 20, 21
Flatworms 22
Fluctuations 12
Forked bristles 106
Fowl 76
Fused veins 107, 108
Galton 154
Geneticist 26
Germ-plasm 142
Geoifroy St. Hiliare 27
Giant 114
Gill-slits 20, 21, 23
Groups I, II, III, IV, 100-118
Haeckel 15
Haemophilia 76
Heliotropism 106, 107
Himalyan rabbits 83
History 1, 6
Janssens 132
Jaunty wing 111
Jennmgs 161, 181-184, 186
Johannsen 156, 157, 159-161,
166, 182
Lamarck 31-34
Langshan 76
Leaves 147
Leidy 186
Lethal 105
Linkage groups 103
Lizard 23
Localization of factors 118
MacDowell 155, 170, 171
Maeritherium 191
Mammal 16, 21, 23
Man 20, 77, 125, 126
Map of Chromosomes 136
Maroon eye color 114
Mendel 40, 41, 52, 89
Mendelian heredity 39
Mendel's law 41-59, 64, 124
Mendel's second law 52
Mesenchyme cells 22
Mesoderm cells 22
Metaphysician 30
Mice 33, 178
Middleton 183
Miniature wing 108
Mirabilis 42
Modifiers 163, 164, 170, 171
Molluscs 22
Mouse 83
Muller 112, 167
Mutations 35, 39, 84
Nageli 34, 35
Natural Selection 36, 145, 146,
187-194
Digitized by
Googk
INDEX
197
Nisus formativus 34
Non-disjunction 139-142
Notch wing 104-106
Nucleus 91
Origin of Species 35, 145
Orthogenesis 34
Paleontology 24-27
Papilio polytes 63
Papilio turnus 63
Paramecium 181, 182
Paratettix 81
Peach eye color 114
Pea comb 55
Pearl 161
Peas 47
Pigeons 172, 174, 175
Pink eye color 114, 115
Planarian 22
Plymouth Rock 76
Podarke 22
Polar bodies 126
Pole arms 5
Protozoa 181
Pseudo-parthenogenesis 183
Purple eye color 109
Purpose 4
Rabbits 83, 170
Rats 176-180
Reduction division 182
Reproductive cells 96
Ruby eye color 106
Rudimentary organ 116
Rudimentary wing 70, 71, 107
Sable body color 107
Science, definition of, 6
Segregation 41
Selenka 94
Sepia eye color 13, 114
Sex chromosomes 118
Sex linked inheritance 75,
118-130
Sexual dimorphism 62 Zeleny 169
Sheep 33
Single comb 55
Sooty body color 50, 114, 115
Speck 68, 69, 111
Spencer 146
Spermatozoon 91, 98
Stars, evolution of, 6
St. Hiliare 27-30
Strap wing 110, 111
Stumpy wing 11
Sturtevant 75, 143
Stylonychia 183
Survival of the fittest 146
Systematist 85
Tails 33
Tan flies 106, 107
Tetrabelodon 191
Trefoil 111
Truncate wing 111, 112, 167,
168
Unfolding principle 34
Unio 22
Unit character 72, 73
Use 31
Variation, discontinuous 13
Vermilion eye color 108, 163
Vestigial wing 11, 55, 56, 109,
133
Vital force 34
Wallace 36
Walnut comb 55
Weismann 17, 31-33
Wilson, E. B. 125
Wingless 67
Winiwarter 126
White eye color 13, 75, 119-130
Whitmg eye color 163, 164
Woodruff 183
Yellow body color 108, 127-129
Yolk sac 16, 17
Digitized by
Googk
Digitized by VjOOQIC
Digitized by
Googk
Digitized by VjOOQIC
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